Mill, and Adams' Commercial Geography are almost indispensable. It should go without saying, of course, that every class-room for the scientific teaching of geography should be supplied with a globe, up-to-date maps, atlases, gazetteers and current geographical magazines.
Ge′olog′ical Survey, United States, is a bureau that was organized by the Department of the Interior in 1879 to bring together the scattered surveys of many western districts and construct geological maps of the United States. The bureau continues to pursue its investigations concerning the products of the country, its mineral resources and the means and possibilities of irrigation. The maps are made in such detail that the scale is never less than four miles to the inch; and is often as large as one mile to the inch. The heights of places are shown by lines, called contours, which represent an equal distance above sea-level. The maps, as they are made, are at once published in folios. The director of the survey issues an annual report. Full information upon the work of the bureau may be gained from the reports of the director, together with Walcott's The United States Geological Survey, Washington, 1895.
Geol′ogy. 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.
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, con-
