glomerate, 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.
Geologic time is divided into five eras and most of these are divided into several periods, as shown in the following table:
| ERAS | PERIODS | ||||
| Cenozoic | Pleistocene (Glacial) | ||||
| |||||
| Mesozoic | Cretaceous | ||||
| Jurassic | |||||
| Triassic | |||||
| Paleozoic | Permian | ||||
| Carboniferous | |||||
| Devonian | |||||
| Silurian ( = Upper Silurian) | |||||
| Ordovician ( = Lower Silurian) | |||||
| Cambrian | |||||
| Proterozoic | Algonkian | ||||
| Archean | Archean | ||||
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
