Derivation of the Quartz.-A careful examination of sedimentary and igneous rocks, even without a microscope, will show that some of the materials are identical in the two. This is especially true of quartz, the material of which sand grains are made. Granite, which is one of the most abundant igneous rocks, is made up of grains of three minerals, quartz, feldspar, and a dark mineral, commonly black mica. Generally these grains are much larger than sand grains and of irregular form. On a broken surface the grains of quartz look like bits of glass.

When granite is exposed to the weather, and especially to moisture in the ground, the other minerals decay but quartz is almost proof against all those agencies which cause other minerals at the surface to decay. When the other minerals decay the rock disintegrates and the quartz grains fall apart. They may be broken up into smaller and smaller particles by alternate beating and cooling.

How Grains are formed and carried to the sea. - Sooner or later these bits of quartz are washed into some stream and the water dashes them against one another and against other stones, breaking them up smaller and sorting them out according to size. The fragments of quartz within a certain range of size are then called sand. At first the fragments are very angular and the sand is then called "sharp." As they are washed down stream the grains are subject not only to breaking but to the rounding off of their corners by grinding on one another. Most sand, however, which has been subject only to stream action is still relatively sharp.

Experience of Sand on the Beach .-After reaching the sea, sand is subject to the action of the breakers (Fig. 13). These may lash it to and fro on the beach for hundreds or even thousands of years. With the breaking of each wave the grain is washed upward on the beach, and in the interval between two breakers it rolls or washes back. Generally the breakers are coming in at an angle to the shore, so instead of being carried straight in at right angles to the shore, the grain is carried in obliquely but it slips out again at right angles. Thus it is dashed in and washed out continually, moving all the time along the shore in a zigzag line. In this ceaseless agitation by the waves, a grain of sand may soon suffer more wear and rounding than in its entire journey by river to the sea. Beach sand is therefore less sharp than river sand derived directly from igneous rocks.

The experience of sand on the beach may be further complicated by wind. If much sand is blown toward the land it may pile up into dunes or traveling hills of sand. Grains of sand thus blown about wear one another more rapidly than when dashed one against the other in water. Dune sand is therefore still rounder (less sharp) than beach sand.

Sand Carried Out by the Undertow.-The water which travels toward the shore in a breaker travels out again at the bottom. This outward current at the bottom is called undertow (Fig. 14). Except under peculiar circumstances it is always present where there are breakers. From time to time, portions of the beach sand are caught in the outward current of the undertow and carried seaward on the bottom. Much dune sand shares the same fate, for the wind does not always blow toward the land. The sand dragged out by the undertow is spread out in a sheet or bed extending outward from the shore to a line where the undertow becomes too weak to carry it farther. This depends on the depth; the farther from shore and the deeper the water, the weaker the undertow. Generally sand is not carried out more than a few miles from shore; in exceptional cases a few dozen miles.

Deposits of Gravel.-From the same reasoning it follows that gravel must be still more narrowly restricted to the shallow water near the edge. The amount of agitation required in order to handle gravel is necessarily greater than in the case of sand, and the undertow, which weakens as it goes seaward, must drop the gravel first.

Much broader sheets of sand and gravel may be formed if the land is gradually sinking and the shore shifting. Then the zone of depositing sand and gravel shifts to follow the moving shore. When a bed of gravel or even of sand is thus made several hundred miles wide, it is evident that it was not all laid down at the same time, but the edge farthest from land was laid down first when the shore was farther out.

Where the Ohio and Miami Derive Their Sand.-In thinking of the vast quantities of sand carried by the Ohio and Miami rivers as on its way to the sea to form sandstone, it should be remembered that most of the sand carried by the Ohio is not derived first-hand from the igneous rocks. Most of the sand which this river carries comes from the breaking down of other sandstones, and the same grains may once have constituted still older sandstones. On the other hand, much of the sand now being carried by the Miami comes from recent glacial deposits, the glaciers having derived it directly from the igneous rocks of Canada. Some of it came down as sand, other parts were delivered to Ohio as bowlders of igneous rock containing quartz. These rocks are now decomposing and yielding sand to the Miami in the manner described above. The Ottawa and some other Canadian rivers carry a still larger proportion of quartz grains which are making their first journey to the sea.

Calcareous Cement.-A bed of sand thus made is converted into sandstone by cementing the grains together. This is illustrated artificially in making common wall plaster in which the grains are cemented together by carbonate of lime (calcite). Some natural sandstone is cemented in the same way, for many natural waters carry much lime in solution. If such water percolates through sand, the lime may be precipitated on the grains, coating them and causing them to adhere. The sands and gravels which partly fill the great valleys in this region are locally affected in this way, making hard plates or beds of sandstone lying between other beds which are still loose and incoherent. Often the cementing process, instead of affecting a whole bed, affects a spheroidal mass of sand or an irregular or curiously shaped mass. Such bodies are called sandstone concretions and are found embedded in the loose sand. The high bank extending east and west from Batavia Junction (Claire Station) south of Madisonville is one of the many places in this region where such curious concretions are found.

Gravel is even more frequently found cemented in this manner. There are few gravel pits in the area in which this tendency is not illustrated; sometimes so abundantly as to interfere seriously with excavation, as in some of the Norfolk and Western Railway gravel pits on the south side of the Little Miami opposite Terrace Park. Plate I-B illustrates the same feature in the pit at Linwood.

Iron Oxide as Cement.-Sand and (more especially) gravel may likewise be cemented by oxide of iron (iron rust). The iron is carried in percolating water in soluble form just as calcite is. When it is oxidized it becomes insoluble and is necessarily precipitated. This coats the grains or pebbles, causing them to adhere thus forming either beds or concretions of a brown color. These are not quite so common in this region as those cemented by calcite but they are not infrequent. Many concretions of this kind are picked up for "petrified potatoes," "petrified walnuts" and the like. Concretions of curious shape are also frequently mistaken for the work of Indians or Mound Builders.

Siliceous Cement.-The above illustrations of cementation by calcite and iron oxide are taken from local and superficial deposits on the land. The great beds of sandstone made beneath the sea are generally cemented by silica, which is chemically the same material as the quartz which constitutes the grains. There may be calcite enough mixed in to make the sandstone effervesce with acid, or iron oxide enough to make the stone brown, red, or yellow, but generally the real cement is silica. It is introduced in the same way as the other cements described.

The cementation of extensive beds is usually done after such beds have been buried by others. This is one reason why lake deposits and river deposits are less frequently converted into rock. Such deposits stand less chance of being well buried and more chance of being eroded away promptly.

Minerals Which Decompose.-It has been pointed out that sand comes from that constituent of igneous rocks which does not decay but merely breaks up. On the other hand, most of the minerals in igneous rocks are subject both to breaking up and to chemical decomposition. Far the most abundant of these constituents is feldspar. It is that mineral which constitutes the larger part of granite and of many other igneous rocks. If the quartz in granite is clearly recognized, then the feldspar is practically all that is left and not black. So far as making sedimentary rocks is concerned, feldspar is the great representative of the decomposable minerals. It is generally among the first to decompose and when this is done the remaining minerals fall apart and the rock is disintegrated.

Kaolin or Pure Clay.-The most abundant substance resulting from decomposition is kaolin. This may be called pure clay. In its pure form (pure potter's clay) it is white and generally very plastic and has no grit even when taken between the teeth. It is rarely found in this condition. Even a slight color indicates some impurity. But though rarely found pure it is, next to quartz, the most wide spread mineral in sedimentary rocks, for it is the basis and characteristic constituent of all clays, and clay (or its consolidated form shale) is much more abundant than any other sedimentary rock.

Even pure clay is made up of individual grains, though very small, often of microscopic dimensions and in some cases too small even for the microscope (colloidal clay).

Mud in Streams.-When rock is once disintegrated, the decomposed and decomposable minerals are washed into streams along with the quartz and started for the sea, being carried as pebbles, sand, and mud. That which is classed as mud consists of the kaolin together with very small particles of any or all other minerals, including very small particles of quartz; in fact these are usually abundant. The mud which settles from the water of the Ohio or Miami is in many cases nearly one-half quartz.

It may be difficult to draw a consistent theoretical line between sand and mud but the following distinctions are useful. Sand consists essentially of quartz; clay, of kaolin with minute particles of any or all other minerals. Sand does not make quiet water turbid, but falls to the bottom; mud remains suspended, settling slowly and meantime clouding the water. When dry, sand is incoherent; mud forms clods or, if pulverized, dust. Sand falls through air; dust settles. Mud is a physical term and does not indicate any definite chemical composition. Mud once deposited in a geological formation is generally called clay.
As with the sand carried by the Ohio and Miami, so with their great burdens of mud, much of it is not derived directly from the decomposition of igneous rocks, but from older sedimentaries. It is not now possible to say how many similar journeys it has made to the sea, but originally it started from the igneous rocks.

Deposition of Mud in the Sea.- When mud reaches the sea is not, like sand, dashed to and fro by breakers, for the simple reason that, if so churned up, it remains in suspension. For the same reason it does not form beaches. Rarely even does it form the bottom in very shallow water which is agitated to the bottom. Remaining in suspension, it is carried outward where the water is quieter at the bottom because deeper. Here it has a chance to settle. If found on shallow and agitated bottoms, it indicates a constant and probably abundant source of supply, for the effect of agitation is constantly to move the mud to deeper and quieter water.

The above statements refer to the margin of the ocean where shallow water passes gradually to deep water. In enclosed bays or lagoons, however shallow, mud must remain and accumulate.

In all the above statements the word clay may be substituted for mud when not thought of in relation to the water which transports it. The brick clays of the Ohio and Miami flood plains are nothing but the mud once carried by those streams.

Change of Clay to Shale.-Shale differs from clay only in being consolidated into a firm rock. All the soft blue beds between the limestones in the quarries of this region are shale. Commonly it is called clay, sometimes incorrectly called soapstone. In an old exposure it is, indeed, generally again decomposed into clay, but where excavation reaches the fresh blue rock this shale is found to be a firm though not enduring rock.

Consolidation of clay into shale may be effected to some extent by cementing the minute particles together just as in the case of sandstone. A spectacular illustration of this is found in clay concretions, some of which assume very curious and even puzzling forms. But an important factor in the consolidation of clay into shale is pressure, just as two pieces of brick clay may be molded into a single mass by pressure. In the making of shale, therefore, it is important that the beds should be well buried by other beds. There is no extensive consolidation of our recent clays in this region corresponding to our prominent conglomerate and sandstone concretions found in the sand and gravel terraces of our larger valleys.

Source of Lime for Marine Shells.-The derivation of limestone from the original rocks of the earth's crust is by a more roundabout process. Most of the igneous rocks contain some lime but only in chemical union with other substances. It is freed from these only by decomposition, and then the lime is dissolved in water. In that way it passes to the sea. There it is abstracted from the water to form the shells and other limy parts of marine animals. Much lime is abstracted by some marine plants, especially algae. Were it not for this process the ocean water would contain, if that were possible, many times more carbonate of lime than it now contains of common salt.

These marine animals live and die in countless numbers, their limy shells accumulating on the bottom. Most of such life is in relatively shallow water (not over 600 feet deep) and hence within a few scores of miles, or at most a few hundred miles, from shore. This relatively shallow area is called the continental shelf because the descent to deep water beyond it is relatively steep (see Fig. 15).

Ooze and Marl.-Shells thus accumulated on the bottom are subject to disintegration. Where the water is very shallow they may be merely broken up by the agitation of the water. Out farther, where the water is deeper they may disintegrate in other ways. In any case they generally make a muddy or pulpy mass called ooze. It should not be forgotten that many of the so-called shells are tests of very minute organisms which do not live on the bottom, but swarm in countless numbers near the surface of the water. After their death their limy tests fall to the bottom. Hence much of the ooze is fine grained or muddy even without further disintegration. The extent to which plants aid in making such accumulations is undetermined.

If the sea bottom be lifted up and the ooze exposed in its unconsolidated condition it is an earthy mass called marl. It is rarely pure, generally containing a small or large admixture of clay.

Consolidation Forming Limestone.-If the ooze instead of being exposed by uplift is buried under later sediment, and time be allowed, it consolidates into limestone. The water which permeates it is strongly charged with lime in solution, that is, it is very hard. This lime in the water is partly precipitated, coating the particles and causing them to adhere as in the case of sandstone.

As thus described (and this is the most common case) the resulting limestone would be fine grained or dense like a few of the beds of the region here considered, or like the Dayton limestone, much used for building in the Miami Valley and Cincinnati. Often it happens, however, that whole shells or large fragments are incorporated. These then become fossils, so abundantly seen in most of our limestones. Again, it often happens that most of the mass is dissolved and again precipitated, not at one time but slowly and. progressively. Thus the limestone becomes a mass of small closely crowded or packed crystals and may be coarse in texture. Such a limestone when freshly broken reflects the light from numerous small facets. This condition is very common in most of the beds which are quarried in this region.

A study of sediment forming at the present time shows that every change in circumstance or condition makes a corresponding change in the nature of the sediments or their structure. Reasoning the other way, it is clear that the great variety of features in our bed rocks give the clue to the circumstances under which they were laid down.

The first and most important principle in this connection is that sediment is, and always has been, as widespread as the sea and, conversely, wherever sedimentary rocks of a given age are found, there the sea was at that time. Wherever sedimentary rocks of that age are absent, there was land at that time, or those sediments were eroded away in a subsequent land period. In this manner the map of land and water must be constructed for each epoch of geologic history. It will be found that shore lines have rarely stood still for any great time, as indeed most of them are not now standing still. In the main the present deep oceans (beyond the continental shelf) have always been ocean, but most of the present area of land and shallow ocean have afforded a ceaseless change of pattern. A detailed study of the rocks in the Cincinnati area affords many illustrations of such shifting.

What Conglomerates and Sandstones Indicate as to Nearness of Shore.-ln determining the conditions of former geologic times, the most important contrast among rocks has to do with the size of the particles which compose them. The coarsest of all maritime rocks are the conglomerates which are merely cemented gravels, the individual stones sometimes being bowlders rather than pebbles. It is plain that all such were deposited at or near the edge of the water. If a bed of conglomerate be 100 miles square, the shore must have shifted during its making. The original bed of gravel began as a narrow strip and grew laterally as in the weaving of a rug. This occurs when the land is sinking. The outer or seaward edge of the conglomerate is older in point of time than the landward edge. By the time the latter was made, the former was already far from shore and covered by finer sediments. Strictly speaking the so-called broad bed or stratum of conglomerate is not a single bed, but the landward edges of many overlapping beds.

Such a formation is called a basal conglomerate because it marks the beginning of a new series of sediments after the area has been for some time out of the water. The whole of it is laid down near shore. Its pebbles are derived from the rocks which were being submerged. The principle here explained is an important one even in this area where there are no true conglomerates, because, as will be seen below, there are some beds consisting of shell fragments which indicate the same thing.

Sandstone does not necessarily indicate such close proximity to the old shore as conglomerate. It may and often does reach to the very shore, but it may also extend much farther seaward. Nevertheless, if a sandstone formation is very broad, say several hundred miles, the presumption is that the shore was shifted during its deposition. Frequently it happens that the same formation is a sandstone at one place and a few miles away a conglomerate. In such a case it may generally be inferred that in passing from the former to the latter, the old shoreline is being approached; but there are also other, less common ways in which the same effect may be produced.

What Shale Indicates as to Nearness of Shore.- Shale, on the whole, was made farther from shore than sandstone. This will be clear when it is remembered that mud is carried out in suspension instead of dragged out along the bottom by the undertow. There is no definite limit to the distance which it may be carried, but most of it settles on the continental shelf except where that is very narrow. With greater distance from land, the process of deposition becomes slower and the beds thinner. Under special circumstances mud may be deposited very near the shore, as in a well protected bay, or where the supply of mud from rivers is excessive.

So far as clastic rocks are concerned, therefore, it is a general principle that (other things equal) the distance from shore varies inversely with the size of the particles, that is, with the coarseness of the sediments.

What Limestone Indicates as to Nearness of Shore.-In a very general way it may be assumed that limestone is made still farther out from shore, but this principle cannot be applied as a rule-of-thumb. Most of the shallow water of the ocean margin has some animal life though it varies greatly in abundance from place to place. Wherever life is, its calcareous remains tend to accumulate, but if waste from the land is accumulating more rapidly at the same place, then the calcareous matter will simply form an ingredient of the clastic beds and will not make limestone. It is only where sediment from the land is very small or absent, or where the remains of life are superabundant, that the latter make limestone. It is purely a matter of proportions.

It is plain then that limestone has the best chance to form at a considerable distance from land, beyond the limits of abundant mud. On the other hand, if little or no sediment is coming from the land, limestone may be formed to the water's edge. This occurred repeatedly in the history of the Cincinnati region. Some limestones here consist entirely of shell fragments broken up and washed about near shore like sand or gravel. In certain cases it appears that older shell limestones had been raised above water and, while sinking again, the waves beat to pieces the older formation and the fragments were again deposited, making a new bed of later age. This new bed is in principle a basal conglomerate, though it is not generally so called because of the confusion which might arise from calling the same rock both a limestone and a conglomerate. The coarse fragmental limestone at the top of the Trenton or base of the Utica shales is believed to be of this character. Likewise the similar limestone at the top of the Eden shales or base of the Fairview.

Gradation Among Different Kinds of Sedimentary Rocks.-Since the proportion of mud to calcareous matter increases gradually toward shore, it is plain that a formation may be pure limestone at one place, argillaceous (clayey) limestone at another, and shale at another. The same formation may likewise grade from shale into sandstone. There are no hard and fixed limits between these types. The Eden shale of the Cincinnati region, when traced southward into Kentucky, becomes locally a sandstone.

Features of Sandstone.-An examination of the grains of a sandstone reveals various circumstances of the time. They may be coarse and angular indicating little or no wear. This is especially significant when fragments of feldspar are mingled with those of quartz. Ordinarily feldspar decomposes or is comminuted to mud before reaching the sea. Its survival means that it came from near at hand or that the climate of that time was arid, thus favoring the disintegration of rocks but not their decomposition. Some such rocks were "subaerial" sediment, that is, they were deposited on the land as stream sediments.

The upper surface of a bed may be ripple marked indicating very shallow water (Fig. 17). A thick horizontal bed of sandstone may be made up of thin oblique laminae (cross bedding) indicating that the bed grew laterally after the manner of a sand bar in a current (Fig. 18). This again indicates very shallow water. Even the color is significant, red and brown sandstones generally being deposited in an arid climate, since abundant decaying vegetation tends to decompose the iron oxide which gives the color.

Features of Shale.-The local conditions attending the deposition of a shale leave corresponding records. Shales perfectly free from grit must have been laid down in very quiet water. Black shales are colored by carbonaceous matter and therefore indicate the presence of life, usually vegetable. Mud cracks are often well preserved and point to the same circumstance which causes mud cracks now, namely, alternate flooding and drying out, probably in flats along the shore alternately submerged and laid bare by tides.

Features of Limestone.- Some of these same features are present in limestone to indicate the circumstances of its making. It may be cross-bedded though this feature is not common. it may also be ripplemarked. A large size ripple, two to four feet between crests, is not uncommon in this region (Fig. 19). The ridges or crests of these ripples are always made of fragments of shells, evidently swept up into ridges on a shallow bottom. Pure limestone indicates a clear sea and argillaceous limestone, a sea more or less turbid.

Alternation of Sediments.-A change from one character of sediment to another indicates some geological event or change of conditions. A single section exposed in sinking a shaft may pass through all the kinds of rock here mentioned and, it may be, coal beside. If a stratum of sandstone is overlain by one of shale, and that by a stratum of limestone, the succession may indicate either one of two processes. The place may have been near shore when the sand was deposited and gradual sinking may have deepened the water, at the same time removing the shore, causing the sediment from the land to become finer in grade and less in amount until it was greatly exceeded by organic remains.

With no movement of the water level and no change in the position of the shore, the same progressive change in sediment may be brought about in another way. Assume the land at first to have been high enough to give the streams sufficient power to carry down such sand to the sea. With the progress of erosion, the land becomes lower, the streams less steep and swift. At length they carry only mud. continued erosion the land is reduced approximately to sea level and the streams are almost without fall and without sediment. The adjacent sea is clear, and calcareous remains predominate over terrigenous sediment up to the very shore.

These two processes may be combined or may be complicated by the assumption of warping, raising land here and depressing it there and changing the whole map of land and water. Further complexities may be introduced by assuming the climate to change, also by assuming ocean currents to shift.

The operation of the two chief factors here named is shown in the following table:

The rocks of the Cincinnati region present a remarkable illustration of frequent alternation between terrigenous sediment and organic accumulation. Doubtless crustal movements, erosion cycles, and the shifting of ocean currents all played their parts in causing the frequent shifts from the one condition to the other. Climatic oscillations may also have been important. The almost rhythmic change at short intervals suggests a possible relationship to certain oscillations of climate. Rhythmic recurrence at short intervals is not known to be characteristic of the other factors named.

What Constitutes a Fossil.-A fossil is any evidence of life in former geologic time. As known in this region such evidence is usually found in petrified sea animals especially their shells. It is not necessary however, that remains be petrified. The younger rocks furnish many fossils whose substance is the original material of shells, bones, or even wood, preserved from decay. Even the hair and flesh of animals of the Glacial Period preserved in the ice of the far north are fossils. The animals or plants may belong to species still living, but the word fossil should not be used for remains of existing species taken from deposits which are still forming. Many fossils are mere impressions such as footprints, or the sediment which filled shells and was preserved as casts.

Petrifaction.-As stated above, the fossils abounding in this region are petrifactions. Despite the fact that limestone is largely made of shells, the present substance of any particular fossil shell is not that which constituted the living shell. If anything of that remains it is merely incidental. The water which once percolated slowly through the mass has removed the original substance of both shell and flesh, molecule by molecule, and substituted for it the carbonate of lime which it carried in solution. The distinguishing feature of petrifaction is that the form is preserved while the substance is changed. Even the structure is sometimes preserved, as in case of the cells in petrified wood. While petrifactions in limestone are most common, the material is often silica, as in most petrified wood. It may be various other substances which abound in mineral veins.

In order that a shell may be fossilized it is necessary that it be incorporated into the sediment and protected from such decay as disintegrates the rocks and makes soil. It must therefore be buried by other beds. Its chances of survival are small unless the bed is then consolidated.

Scientific Value of Fossils.-The most evident use to which fossils may be put is in the study of the evolution of forms of life. Fossils of related species may thus be arranged in series according to the line of descent. Those which come from the lower rocks necessarily represent the earlier forms. On the whole the earlier are simpler and the later forms are more complex and higher. By arranging species in the order of time the principles may be seen, according to which development proceeded.

Practical Use of Fossils.-The great practical use of fossils depends on the fact that types of life have not only come but gone. All are more or less temporary. Geological and biological evidences go to show that each type came into existence but once and, having become extinct, never returned. All rocks containing one type of fossil must have been made while that form was living. It is assumed that beds containing fossils of the same species were deposited at the same time. Thus rocks, the world over, are classified and correlated according to the time in which they were made.

This general principle would be easily applied if each species had lived only during the making of a single formation. Some of them did, but others continued to thrive throughout the making of dozens of formations. Moreover animals migrated in colonies then just as they do now. This took time, and at the place where they first bred they are found in older deposits than at points which were reached after slow and lengthy migrations. But despite such complications the rocks underlying civilized countries are coming to be classified as to age, with a considerable degree of accuracy and consistency, so that rocks of a given age may be known by their fossils wherever found, despite their differences in physical character.

Fossils may be considered as labels on the rocks in which they occur, stating their age and their relative position above or below other rocks in the column. Without the study of fossils the science of Geology and its practical application would still be in a very undeveloped condition.

Various Units Defined.-In the description and discussion of sedimentary rocks it is impossible to describe each layer separately or to delineate it on a map. It is generally necessary that a large number of layers be treated together as a unit as in the case of our limestones and shales. The terms used in such grouping have technical meanings and must not be confused, The 'word bed is used by geologists in the same sense as by quarrymen. It implies that the rock separates along certain planes, due to the manner of its deposition. These planes are called bedding planes and the thickness of rock which is thus easily separated from its neighbors is called a bed. Adjacent beds may be of different material, as is commonly the case here where limestone alternates with shale; but many quarries show bed after bed of the same rock easily separated. The word stratum (plural strata) applies either to one bed or many beds in contact, so long as all are of the same material. In other words, a stratum must not be separable into beds of two distinct kinds. In this vicinity the word stratum would mean about the same as bed, since single beds of limestone usually alternate with beds of shale.

Generally a great number of beds, and often of strata, are taken together for purposes of discussion and geologic mapping. The larger unit thus made is called a formation. This is the most essential term in the grouping of beds and it should not be used in any other sense. On detailed geologic maps each formation is distinguished by a separate color or symbol. It may sometimes be subdivided into two or more members, or it may consist of a single stratum or even a single bed. Division into formations is universal, but in many cases there is no need for division into members. Just as the dollar is the unit of our currency, though fractional parts are much used, so the remains the essential unit for sedimentary rocks though it may be subdivided into members as is done with most of the formations in this area.

The most important unit larger than the formation is the system. Parts of two systems are shown in this area, the Ordovician and the Silurian. The meaning of the terms series and group is sufficiently clear from their use in the table II and in the discussion which follows.

Terms Denoting Divisions of Time.-Two names indicating units of geologic time are also important. Period is the time name corresponding to a system of stratified rocks. Those of this area were deposited in the Ordovician and Silurian periods. The word epoch denotes a shorter lapse of time. It may be the time during which a formation or a series was made, or a time in which the region was above water and no sediments were forming.

It has thus far been assumed that all beds were laid down one after another in horizontal position and separated from each other only by bedding planes. Beds thus laid down are said to be conformable. Frequently, however, such rocks are raised above the sea and eroded. When the land sinks again beneath the sea and receives new sediments, the new beds are separated from the old, not by a bedding plane but by an erosion surface. The two series of beds are then said to be unconformable. Figure 20a shows unconformity of angle, not illustrated in this area. Figure 20b shows unconformity of erosion only. It is exemplified in this area by the relation between the, glacial formations and the older bed rocks.

Another type of unconformity known as overlap is of special interest in this area. It may be described as follows:
All the formations known in any one area constitute the column of rocks for that area. - Such a column for this area is shown in Table II. In another place 50 miles distant in any direction, the column of Ordovician and Silurian rocks would be somewhat different. Some formations present in one area would be absent from the others. This is because there were epochs in which the one area was sea and the other land. If three or four formations were made during the time that the land was sinking beneath the sea, each successive formation would overlap its predecessor. The exposure of a complete section at any one place might give no hint of an unconformity, that is it might not show that any formations were missing until compared with sections from other places. In this area and adjacent areas a number of formations or members are seen to overlap their predecessors. In many cases this is the only means of knowing that the locality was land for a part of the time.

It is the function of a geologic map to show what formation immediately underlies the mantle rock at every place. This is done by using a different color for each formation. The mantle rock is ignored (note 4). If the strata are horizontal the highest in the column must occupy the highest ground. On the other hand if the surface is nearly horizontal and the strata are inclined or dipping, the direction of dip will be from the outcrop of the older toward that of the younger formations. A study of figures 21-26 will make these statements clear.

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*Sometimes it is need to make two geologic maps of the same area, one showing the several formations of the bed rock and another showing the unconsolidated formations included in the mantle rock. The first is illustrated by figure 3, The second by the physiographic map (in pocket).

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