STRATIGRAPHICAL GEOLOGY arranges the rocks of the earth's crust in the order of their appearance, and interprets the sequence of events of which they form the records. Its province is to cull from all the other departments of geology the facts which may be needed to show what has been the progress of our planet, and of each continent and country, from the earliest times of which the rocks have preserved any memorial. Thus from mineralogy and petrography it obtains information regarding the origin and subsequent mutations of minerals and rocks; from dynamical geology it learns by what agencies the materials of the earth's crust have been formed, altered, broken, upheaved, and melted; from structural geology it understands how these materials were put together so as to build up the complicated crust of the earth; from paleontological geology it receives. in well-determined fossil remains, a clew by which to discriminate the different stratified formations and to trace the grand onward march of organized existence upon this planet. Though the geological record is at best but an imperfect chronicle of the history of the ‘:arth, from this record alone can the progress of the globe be traced. It contains the registers of the births and deaths of tribes of plants and animals which have from time to time lived on our planet. But a small proportion of the total num ber of species which have appeared in past time have been thus chronicled, yet by col lecting the broken fragments of the record an outline at least of the history of life upon the earth can be deciphered. The nomenclatures adopted for the subdivisions of the geological record bear witness to the rapid growth of geology. It is a patch-work in which no system nor language has been adhered to, but where the influence by which the progress of the science has been molded may be distinctly traced. Some of the earliest names are lithological, and remind us of the fact that mineralogy and petrography preceded geology in the order of birth—chalk, oolite, greensand, millstone grit. Others are topographical, and often recall the labors of the early geologists of Ilngland —London clay, Oxford clay, Purbeck, Portland, Kimmeridge beds. Others arc taken from local English provincial names—Lias, Gault, Crag, Cornbrash. Others of later date recognize an order of superposition as already established among formations —old red sandstone, new red sandstone. By common consent it is taltryitted that names taken from the region where a formation or group of rocks is typically developed, are best adapted for general use. Cambrian, Devonian, Silurian, Permian, Jurassic, are of this class, and have been adopted all over the globe. The geological record is classified into five main divisions: 1. The archaean, azoic (lifeless), or eozoic (dawn of life) periods; 2. The primary or paleozoic (ancient life) periods; 3. The secondary or mesozoic (middle life) periods; 4. The tertiary or post-tertiary periods. These divisions are further arranged into systems, each system into formations, each formation into groups, and each group or series into single zones or horizons. The subjoined general ized table exhibits the order in which the chief subdivisions appear: PnvsfouRAPincAr, GEOLOGY deals chiefly with the surface of the earth and the changes thereon. When the geologist asks himself bow the present distribution of sea and land is to be accounted for, be finds that the answer of the question goes back to early paleozoic times, whence he can in some cases trace the growth of a continent downward through the long cycles of geological time. But there still remains the problem of the original wrinkling of the surface of the globe, whereby the present great ridges and hollows were produced. It is generally agreed that these inequalities have been produced by unequal contraction of the earth's mass, the interior contracting more than the outer crust, which must therefore have accommodated itself to this diminution of diameter by undergoing corrugation. But there seems to have been some original distribution of materials in the globe that initiated the depressions on the areas which they have retained. The matter underlying the oceans is more dense than that beneath the continents, and to this cause, in part at least, must the present position of the oceans be attributed. The early and persistent subsidence of these areas, with the consequent increase of density, seems to have determined the main contours of the earth's surface. Rocks which were originally horizontal, or nearly so, have been crumpled over tracts thousands of square miles in extent, so as to occupy a superficial area greatly less than that which they originally covered. It is evident that they have been horizontally com pressed, and that this result can have been achieved only as a consequence of the sub sidence of such a curved surface as that of our globe. One writer of eminence sup poses that the present inequalities of contour on the earth's surface are from 11+ to 66 times as great as they would have been had they resulted from the contraction of a 'solid globe; and he has suggested that the earth need not have become solid throughout simultaneously, and consequently may have been considerably larger at the time when a solid crust was formed than it is now. The theory of a hot fluid interior of the earth, so long and so resolutely held by geologists as well as laymen, has in recent times been weightily opposed. But it is the surface and not the interior that we are now consider ing. The hypothesis of secular cooling and contraction furnishes a natural explanation of the origin orthe dominant elevations and depressions on the surface, and of the intense crumpling which the rocks in many regions have undergone. Taking 0.09 as the coefficient of contraction for a supposed stratum 500 m. thick, lying beneath 25 in. of crust, and passing from a fused into a solid state, one, investigator concludes that every hundred miles measured along a great circle on the surface would have been one mile longer before the contraction, and that this might produce a triangular elevation of 25 sq.m. on a base of 100 m., which would give a range of mountains half a mile high. If only 50 m. out of the 100 were disturbed, the range would be a mile high, and so on. The effects of this lateral pressure may show themselves either in broad dome like elevations, or in narrower and loftier ridges of mountain. The structure of the crust is so complex and the resistance offered by it to the pressure is consequently so varied, that abundant cause is furnished for almost any diversity in the forms and dis tribution of the wrinkles into which it is thrown. It is evident, however, that the folds have tended to follow a linear direction. In North America, from early geological times, they have kept on the whole on the lines of meridians of longtitude; in the old world they have chosen diverse trends, but the last great crumplings—those of the Alps, Cau casus, and the great mountain ranges of central Asia—have risen along parallels of latitude. Mountain chains must, therefore, be regarded as evidence of the shrinkage of the earth's mass. The theory of secular contraction serves also to show why vol canoes so frequently rise along the mountain ridges. The elevation of the crust, by dimin ishing the pressure on the parts underneath the upraised tracts, permits them to assume a liquid condition and to rise withiu reach of the surface, when, driven upwards by the expansion of superheated vapors, they are ejected in the form of lava or ashes. But these subterranean movements form only one phase of the operations by which the outlines of the land have been produced. They have ridged up the solid crust above the sea level, and have thus given rise to land; but the land as we now see it has acquired its features from the prolonged and varied action of the epigene agents upon rocks of widely varied heights and powers of resistance. It is evident that as a whole the land suffers ceaseless erosion from the time that it appears above water. It is likewise clear, from the nature of the materials composing most of the rocks of the land, that they have been derived from old denundations of the same kind, and thus, side by side with the various upheavals and subsidences, there has been a. continuous removal of mate rials from the land and an equally persistent deposit of these materials under water, and consequent growth of new rocks. The work of rain, of frost, of rivers, of glaciers can be readily discriminated, though they all combine harmoniously toward the achieve ment of their common task. Taking a broad view of denudation, we may conven iently group together the action of air, frost, springs, rivers, glaciers, and the other agents which wear down the surface of the laud, under the common designation of sub aerial, and that of the sea as marine. The general results of subaerial action are—to furrow and channel the land, to erode valleys, to sharpen and splinter the ridges of mountains, and thus, while roughening, to lower the general surface and carry the detritus out to sea. The action of the sea, on the other hand, is to plane down the land to the level at which the influence of breakers and ground-swell ceases to have any ero sive effect; the flat platform so often visible between tide-masks on a rocky exposed coast-line, is an impressive illustration of the tendency of marine. denudation. The combined result of subaerial and marine action, if unimpeded by any subterranean movement, would evidently be 'to reduce the land to one general level under the sea. But to reduce a continent to the condition of a submarine plain would require a longer period of time than seems to have elapsed between any two epochs of upheaval. An idea of the magnitude of surface denudation is found in the action of the great Missis sippi river, winch, it has been estimated, wears away every year from the vast territory drained by it of a foot. At this rate, and taking Humboldt's estimate of 748 ft. as the average height of the continent, the whole of North America would be worn down to the sea-level in about 4,G00,000 years—a comparatively short period in geological chronology. Hence it follows that, apart altogether from irregularities of surface due to inequalities of upheaval, every. air, of land exposed to ordinary subaerial action must in the end be channeled into a system of valleys. Nor would this require a long geological period, for, at the present rate of waste in the Mississippi basin, valleys 800 ft: deep might be made iu a million years. Undoubtedly the original
.features superinduced by subterranean action would guide and modify the operations of running water, though their influence would certainly wane as the features them selves slowly disappeared. In no case, probably, would the aboriginal contour remain through a succession of geological periods. Traces of it might still be discernible, but they would be well nigh effacted by the new outlines produced by the superficial agents. In the vast table lands of Colorado and the western territories of the United States is an impressive picture of the results of mere subaerial erosion on undisturbed and nearly level strata. Systems of stream-courses, and valleys, river gorges unex ampled elsewhere in the world for depth and length, vast winding lines of escarpment, like ranges of sea-cliffs, terraced slopes rising from plateau to plateau, huge buttresses and solitary stacks standing like islands out of the great mountain masses tower ing into picturesque peaks and pinnacles, cleft by innumerable gullies, yet everywhere marked by the parallel bars of the horizontal strata out of which they have been carved, these arc the orderly symmetrical characteristics of a country where the scenery is due entirely to the action of subaerial agents on the one hand and the varying resistance of perfectly regular stratified rocks on the other. The Alps, on the contrary, present an instructive example of the kind of scenery that arises where a mass of high ground has resulted from the intense corrugation and upheaval of a complicated series of stratified and crystalline rocks, the vast period carved by rain, frost, springs, and glaciers. We see how, on tile outer flanks of those mountains among the ridges of the Jura, the strata begin to undulate in long wave-like ridges, and now, as we enter the main chai', the undulations assume a' more gigantic tumultuous character, until, along the cential heights, the mountains lift themselves towards the sky, like the storm-swept crest of vast earth billows. • The whole aspect of the ground suggests intense commotion. Where the strata appear along the cliffs or slopes, they may often be seen twisted and crumpled on the most gigantic scale. Out of this complicated mass of material, the subaerial forces have been ceaselessly at work since its first elevation. They have cut out valleys, sometimes along the original depressions, sometimes down the slopes. They have eroded lake basins, dug out corries or cirques, notched and furrowed the ridges, splintered the crests, and have left no part of the original surface unmodified. But they have not effaced all traces of the convtilsions in which the Alps were upheaved. The details of the sculpture of the laud have mainly depended on the nature of the materials on which nature's erosive tools have been employed. The joints by which all rocks are traversed have served as dominant lines along which the rain has filtered, and the springs have risen, and the frost wedges have been driven. On the high bare scarps of a high mountain, the inner structure of the mass is laid open, and there the sys tem of joints is seen to have determined the lines of crest, the vertical walls of cliff and precipice, the forms of buttress and recess, the position of cleft and chasm, the outline of spire and pinnacle. On the lower slopes, even under the tapestry of verdure which nature delights to band where she can over her naked rocks, we may d'etect the same pervading influence of the joints upon the forms assumed by ravines and crags. Each kind of rock, too, gives rise to its own characteristic scenery. The massive crystalline rocks, such as granite, yield each in its own fashion to the resistless attacks of the denuding forces. They are broadly marked off from the stratified rocks in which the parallel bands of the bedding form a leading feature in every cliff and bare mountain slope. Among the latter rocks also, very distinctive types of surface may be observed. A range of sandstone hill, for example, presents a marked contrast to one of limestone. In the physiography of any region, the mountains are the dominant features. A true mountain chain consists of rocks have been crumpled and pushed up in the man ner already described. But ranges of hills almost mountainous in their bulk may be formed by the gradual erosion of valleys out of a mass of original high ground. In this may some ancient table-lands, those of Norway and of.the Highlands of Scotland, for example, have been so channeled by deep fjords and glens that they now consist of massive, rugged hills, either isolated or connected along the flanks. The forms of the valleys thus eroded have been governed partly by the structure and composition of the rocks, and partly by the relative potency of the different denuding agents. Where the influence of rain and frost has been slight, and the Streams, .supplied from distant sources, have had sufficient declivity, deep, narrow, precipitous ravines or gorges have been excavated. The canons of the Colorado are a magnificent example of this result. Where, on the other hand, ordinary atmospheric action has been more rapid, the sides of the river channels have been attacked, and open sloping glens and valleys have been hollowed out. A gorge or defile is usually due to the action of a waterfall, which, beginning with some abrupt declivity or precipice in the course of the river when it first commenced to flow, or caused by some had rock crossing the channel, has eaten its way backward. Lakes may have been formed in several ways. 1. By subterranean movements, as, for example, during those which gave rise to mountain chains. But these hollows, unless continually deepened by subsequents movements of a similar nature would be filled up by, the sediment continually washed into them from the adjoin ing slopes. The numerous lakes in such a mountain system as the Alps cannot be due merely to this cause, unless we• suppose the upheaval of the mountains to have been geologically quite recent, or that subsidence must take place continuously or periodically below each independent basin. But there is evidence that the upheaval is not of recent date, while the idea of perpetuating lakes by continual subsidence would demand, not in the Alps merely, but all over the northern hemisphere where lakes are so abundant, an amount of subterranean movement of which, if it really existed, there would assuredly be—as there is not—plenty of other evidence. 2. By irregularities in the deposition of superficial accumulations prior to the elevation of the land or during the disappearance of the ice-sheet. The numerous lakes inclosed within ridges and mounds of drift clay and gravel, are examples. 3. By the accumulation of a barrier across the channel of a stream, and the consequent holding back of the water. This may be done, for instance, by a landslip, by the advance of a glacier across a valley, or by the throwing up of a bank, by the sea, across the mouth of a river. 4. By erosion. The only agent capable of excavating hollows out of the solid rock, such as might form lake-basins, is glacier-ice. It is a remarkable fact, of which the significance may now be seen, that the innumera ble lake-basins of the northern hemisphere lie on surfaces of intensely ice-worn rock. The strim can be seen on the Smoother rock-surfaces running down into the water on all sides. These strim were produced by ice moving over the rock. If the ice could, as the strim prove, descend into the rock basins and mount up the farther side, smoothing and striating the rock as it went, it could erode the basins. It is hardly possible to convey in words an adequate conception of the enormous extent to which the north of Europe and North America have had their surfaces ground down by ice. The ordinary rough surfaces produced by atmospheric disintegration have been replaced by a peculiar flowing contour, which is traceable even to below the sea-level. The table-lands may sometimes arise from the abrasions of and the production of a level plain by the action of the sea, or rather of that action combined with the previous degradation of the land by subaerial waste. But most of the great table-lands of the globe seem to be platforms of little disturbed strata which have been upraised bodily to a considerable elevation. No sooner, however, are they placed in that position than they are attacked by running water, and begin to be hollowed out into systems of valleys. As the valleys sink, the platforms between them grow into narrower and more definite ridges, until eventually the level table-land is converted into a complicated network of hills and val leys, wherein, nevertheless, the key to the whole arrangement is furnished by a knowl edge of the disposition and effects of the flow of water. The examples of this process brought to light in the states of Colorado and Nevada, and in Wyoming and the other western territories, by Newberry, King, Hayden, Powell, and other explorers, are among the most striking monuments of geological operations in the world. The mate rials worn from the surface of the higher are spread out over the lower grounds. We have already traced how streams at once begin to drop their freight of sediment when, by the lessening of their declivity, their carrying power is diminished. The great plains of the earth's surface are due to this deposit of gravel, sand, and loam. They are thus monuments at once of the dsstructive and reproductive processes which have been in progress unceasingly since the first land rose above the sea, and the first shower of rain fell. Every pebble and particle of their soil, once part of the distant mountains, has traveled slowly and fitfully downward. Again and again have these materials been shifted, ever moving downward and seaward. For centuries, perhaps, they have taken their share in the fertility of the plains and have ministered to the necessities of flower and tree, of the bird of the air, the beast of the field, and of man himself. But their destiny is still the great ocean. In that bourne alone can they find undisturbed repose, and there, slowly accumulating in massive beds, they will remain until, in the course of ages, renewed upheaval shall raise them into future land, there once more to pass through the same cycle of change.—[In large part condensed from Encyclopadia Britan nica, 9th edition.]