Geology is the science which deals with the history of the earth. It is the task of geology, not simply to recite the history of the earth so far as it is known, but to show how this history became known and how the limits of knowledge are being extended. Geology is a young science, and in its study at the present time it is needful to take cognizance of the limitations of present knowledge as well as of the knowledge itself. Everything which throws light on the history of the earth falls within the field of geology.
The history of the atmosphere and the history of the ocean are really parts of geology, since the atmosphere and the ocean are parts of the earth. The popular impression, therefore, that geology has to do only with the rocks of the earth is not altogether adequate. The rocks of the earth, to be sure, furnish the larger part of the data for unraveling the history of the earth, though they are not the only sources of information. It is to be remembered, too, that in the study of the rocks, it is study of them for the sake of the light they throw on earth-history, rather than study of them for their own sake, with which the geologist is primarily concerned.
In working out the history of the earth, so far as it has been worked out, the line of approach has been through the study of the changes which are now taking place on the earth's surface. The rain falls on the land, and some of it gathers into streams, and the streams flow into the sea. In the flow of the water the substance of the land is worn away, the material carried to the sea and deposited there in the form of gravel, sand, mud etc. The sand and mud need nothing but cementation to become sandstone and shale, two of the commonest sorts of rock found on the land.
The process of cementation is now going on by natural means in many places. In the sand and the mud, as they are deposited in the sea, shells of various organisms are often imbedded. The shale and sandstone of the land also contain shells and other traces of marine organisms. Hence it is inferred that the sandstone and shale, as well as certain other sorts of rock found on the land, were originally deposited as beds of sand and mud, etc. in the sea and that they have since been elevated to the condition of land.
The activities of other surface-agencies are similarly studied. The detailed study of the work now being done by rain and rivers, underground water, waves and currents, the atmosphere, glaciers, changes of temperature, gravity, organic agencies and all other forces and activities operative on the surface of the earth has taught geologists how to interpret the rocks formed in ages long past. It is by the interpretation of the recorded results of the past, in the light of the processes now taking place, that the science of geology has grown up. The study of present processes is becoming more and more exhaustive, and the application of this increased knowledge of present processes to the records of the past is continually enlarging and perfecting our knowledge of the earth's history.
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Geology should begin with the origin of the earth, and at this point it trenches upon the field of astronomy. The early ages of the earth's history are as yet speculative. There seems to be good reason for doubting the truth of the nebular hypothesis, which has so long been regarded as satisfactory. The only rival hypothesis which has been framed is the meteoric hypothesis, which affirms that the earth is made up of an aggregation of meteorites comparable to the meteorites and shooting stars, which daily reach the earth by millions at the present time. While the stages of the earth's history preceding the beginning of sedimentation are, at the present time, largely conjectural, many lines of investigation are being pursued which ultimately may throw much light on the early and obscure portions of the earth's history. The general outlines of this history since sedimentation began are probably fairly well-understood, though increased knowledge may modify present conceptions at many points.
The rocks of the earth, which contain the principal records of the earth's history are of three great classes: (1) Igneous rocks or those which represent solidified lava; (2) sedimentary rocks, as shale, sandstone, conglomerate, etc., most of which are made up of fragments of older rocks; and (3) metamorphic rocks, which may have been so far altered by various means that they now depart notably from the original forms. In the metamorphism of rocks pressure is the most potent agent. Chemical change, under the influence of moisture, is probably second in importance; and heat third. A special class of sedimentary rocks is due to life. Here belong most limestones, made of shells, corals, etc.; coal, of plant origin; and a number of lesser formations.
The composition, disposition and structure of these several sorts of rock and their fossil contents, so far as they contain fossils, interpreted in the light of processes now taking place, allow geologists to infer the conditions under which the various sorts of rocks are made. When geologists are able to tell what the conditions were on every part of the earth at every period of the past, the science of geology will be complete.
Several subdivisions of geology are often recognized. Dynamic geology deals with the agents and forces which have been concerned in making the earth what it is. To dynamic geology belongs the consideration of the activities of rivers, the atmosphere and the ocean, the forces concerned in volcanic action, crustal movements, etc. Petrography is that phase of geology which deals with the rocks as such. Structural geology is that branch of the subject which deals with the positions of rock-strata and with the relations of rock formations to one another. Structural geology might be defined as the architecture of the crust of the earth. Physiographic geology deals with the forms of the surface, and its task is to explain how the present surface came to be as it is. It draws extensively on dynamic geology, since it must consider the forces which have produced mountains, plateaus and plains as well as the details of their surfaces. It draws on structural geology, because the positions in which the rocks occur influence the shape of the surface at the present time. It draws on petrography, because the form of the surface is often affected by the character of the rock beneath. Paleontologic geology is that phase of the subject which deals with fossils. The objective point of paleontologic geology is to determine the character of the life of successive ages and the changes which it underwent from time to time. Historic geology is that phase of the subject which deals with the application of all other phases of geology to the task of making out the history of the earth, as shown in the rock formations. Economic geology is that phase of the subject which deals with the materials of the earth's crust which are commercially valuable. It has to do with ores of all sorts; with coal; with building-stone; with clays which are valuable for the manufacture of brick, pottery, etc.; with materials which can be used for pigments; sand, which can be used for glass; with precious stones; with abrasive materials; with asphaltum, petroleum, natural gas, salt, fertilizers etc. The function of economic geology is to determine the origin of these substances and, so far as possible, the laws which govern their distribution.
The Archean era was the time occupied in the making of the oldest system of rocks. The Archean rocks are mostly metamorphosed igneous rocks, though with them there are some metamorphic sedimentary rocks. Fossils have not been found in this system of rocks, but it cannot be asserted that life did not exist.
The Proterozoic era is the time during which the great system of rocks lying above the Archean and below the oldest abundant fossiliferous rocks was deposited. The rocks of the Proterozoic era are mainly sedimentary, though igneous rocks have great development locally. The formations of the Proterozoic era are many thousands of feet thick, though considerable portions have been removed by erosion. It has been estimated that the Proterozoic era was perhaps as long as all subsequent time. Life existed during this era, as is shown by the few fossils which have been found and by the nature of some of the formations, even where definite fossils do not occur. For example, there are black shales and graphitic slates, the carbon of which probably is of plant-origin. The Proterozoic (Algonkian) rocks of the Lake Superior region contain rich beds of iron and copper.
The Paleozoic (formerly called Primary) era was the time when the several systems of rocks bearing the names Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian were deposited. The time occupied in the deposition of each of these systems is a period. These systems of rocks are mainly of sedimentary origin, and the materials of which they are composed were derived from the land-areas which existed when these systems were being deposited. These materials were washed down from the land to the sea, and there deposited. The several systems of Paleozoic rocks are distinguished from one another by their fossils. Thus, the fossils of the Cambrian system of rocks are sufficiently unlike those of the Ordovician system to be readily distinguished by paleontologists. Even at the beginning of the Cambrian period the range of life was great, all the great types which are living at the present time, except the vertebrates, being represented, given the vertebrates may have lived, though relics of their existence have never been found. In this period trilobites and brachiopods were perhaps the most abundant and the most characteristic life, judged by the abundance of fossils. In the rocks of the Ordovician system fish-remains have been found and also relics of air-breathing life. Mollusks, crinoids and corals lived in great profusion, in addition to types of life which predominated in the Cambrian period. Most of the oil and gas of Ohio and Indiana come from the rocks of the Ordovician age. In the Silurian period the same general types of life were prevalent, but the species are so far unlike those of the preceding period as to be readily distinguished by those who are familiar with fossils. The Devonian period is often known as the Age of Fishes, on account of the abundance of fish-remains which have been found in the rocks of this system. It is far from certain, however, that fish were more abundant than now, and the variety of fish was probably less than at the present time. The Devonian was, however, the first period when fishes were abundant, so far as now known. The oil of Pennsylvania and Canada is largely derived from beds of the Devonian age. During the Carboniferous period there were extensive marshes in the United States and in some other parts of the world, in which vegetable-matter accumulated in great quantity. These marshes were subsequently submerged, and the vegetable matter was buried by sediments and ultimately converted into coal. Most of the coal of the United States east of the Great Plains was accumulated at this period. Plant-life was abundant, but the plants were largely of types now extinct. It was during this period, as far as now known, that reptilian life began. The Permian period represents a transition stage between the Paleozoic and the Mesozoic era. In the Permian period there was extensive glaciation in Australia, South Africa and India.
The several systems of Paleozoic rocks have somewhat different distribution, and, since the area of the deposits of any period corresponds approximately with the submerged area of that period, the distribution of the several systems helps us to understand the relations of land and water during the several periods. In this way it is known that the relations of sea and land were different at different times. It would appear either that the continent repeatedly oscillated up and down, causing areas which were at one time submerged to rise to above the level of the water at another and areas which were land at one time to sink beneath the sea at another; or else that the sea-level itself fluctuated. If the sea-level rose, it would overspread the low lands; if it were lowered, it would cause areas which had been submerged to become land. How far the changes in geography were the result of land-oscillations and how far they were the result of oscillations of the sea-level has never been determined. So far as present knowledge goes, it would appear that the deep-sea bottom has at no time been land and that the areas which were alternately above and below sea-level were low when they were land, and covered by shallow water only when they were submerged.
The Mesozoic (formerly called secondary) era, as the term indicates, was the era when life intermediate between the ancient and the present existed. This era is divided into several periods, as indicated above. The Triassic formations of North America are somewhat widespread in the western third of the continent, but have but little development in the eastern part and none at all in the interior. During this period reptiles perhaps were the dominant type of life. They were not only numerous, but the individuals attained great size. The earliest remains of mammals date from this period. Marine life abounded, but departed notably from the types which had prevailed in the Paleozoic era. Vegetation was abundant, but of types now extinct or but meagerly represented. The Jurassic period followed, and the distribution of the Jurassic formations is similar to that of the Triassic. The life of this period was, however, somewhat different from that of the preceding, though the same general types abounded. The oldest remains of birds are found in the rocks of this period. The Jurassic period was followed by the Cretaceous or Chalk period. In the early part of this period chalk was not being deposited, but in the later part chalk-deposits were in process of formation in many parts of the earth. The chalk-deposits are made up, for the most part, of the shells of minute marine animals (see Chalk). The Cretaceous formations of North America are much more widespread than those of the Jurassic and Triassic periods. Their distribution indicates that a large part of the North American continent was submerged during some parts of the period. It was during the Cretaceous period that the modern types of plants and of fishes made their appearance. During the last stages of the Cretaceous period extensive accumulations of vegetable-matter were made in the western third of the United States. These were subsequently converted into coal, mostly soft. The coal of the western part of the United States mostly belongs to the late stages of the Cretaceous period.
The Cenozoic era or era of modern life followed the Mesozoic. Mammals, the earliest remains of which are found in the rocks of the Triassic system, abounded during the Cenozoic era, while the huge reptiles which had been especially characteristic of the Mesozoic era had disappeared. Reptiles still existed as now, but they were of relatively small types, and their numbers appear to have been relatively few. As the Cenozoic era progressed, the forms of life approached more and more closely to those of the present time, and by the end of the Pliocene the life was nearly the same as that which now exists. The Pleistocene period was a remarkable one, on account of the great climatic changes which occurred at this time. The result of these climatic changes brought on a glacial climate, and an ice-sheet or series of ice-sheets covered something like 4,000,000 square miles in the northern part of North America. A large ice-sheet was developed, probably contemporaneously, on the continent of Europe, affecting especially its northwestern part. The history of the Glacial period has been worked out in sufficient detail, so that it is known that there was a series of ice-sheets and not a single one. That is, there were several glacial epochs, with intervening epochs when the ice largely or completely disappeared and the climate became genial. Most of the lakes, waterfalls, etc. of the northeastern part of the United States and of the region to the north owe their existence to the glaciation of the area where they occur. The ice reached its most southerly limit in Illinois, where it descended to about 37° 30'.
The duration of the earth's history is a matter which has received much attention, but no conclusions have been reached which can be relied upon, beyond the very general one that the history of the earth has been exceedingly long. Various conjectures as to the number of years occupied in bringing the earth to its present condition have been made. They range from twenty-five million or so to several hundred million. Geologists in general would not be surprised if it could be demonstrated that the history of the earth since Archean time has occupied 50,000,000 to 100,000,000 years. The Proterozoic era was perhaps as long as all subsequent time. The Paleozoic era was perhaps three to six times as long as the Mesozoic, and the Mesozoic was perhaps three or four times as long as the Cenozoic.
The climatic changes which the earth has undergone have been great, but their causes are not well-understood. There is little basis for the belief, formerly widespread, that the climate has on the whole been growing cooler. Cold periods seem to have alternated with warmer ones. Glaciation was extensive at the close of the Paleozoic and, again, late in the Cenozoic, and there is some indication of cold periods at other times. On the other hand the lands of high latitudes enjoyed genial climates during some parts of the earth's history, even as late as the mid-Tertiary time.
Volcanic activity seems to have been greater at some periods than at others, but on the whole to have been about as great, so far as now known, in the early as in the late stages of the earth's history. See Dana's Manual of Geology; Scott's Introduction to Geology; Le Conte's Elements of Geology; Geikie's Text-Book of Geology.
The New Student's Reference Work (1914) pgs. 747-750.