PHYSIOGRAPHIC GEOLOGY This branch of geology is specifically treat ed under the heading of PHYSIOGRAPHY, so that only the general principles will be here stated. Physiography is concerned with a study of the forms assumed by the surface of the crust and the origin of these forms. Both on the ocean bottom and on the continents, plains, mountains, and volcanoes have been built, and each of these crust forms has a history. This history may start with the origin—the plain is an old lake bed, or a raised sea bottom, or a lava plain, etc. After its origin changes of one kind or another have occurred, giving it its present modified characteristics. For example, rivers may have developed upon it, or the agencies of the sea may be at work upon it, or glaciers may have passed over it. It is a question of physio graphic geology to decide what has happened since the origin of a given land form.
In recent years, largely as a result of the work done by Professor Davis, it has been found that land forms normally pass through a life history which can be stated in terms of youth, maturity, and old age. The characteristics of a newly formed coast-line, a young stream valley, or a mature plain are readily seen. These aspects of physiography may be considered briefly by a few examples. A young stream has steep sides, be cause there has not been time enough for weather ing to broaden them; it is certain to have falls or rapids, if the rock texture is irregular, because it has not yet established a grade, and is there fore busily cutting in its bed and discovering rock The fossil organisms, whose study forms the basis of paleontology or biogeology, in connection with a study of the rocks themselves, are useful in telling of past changes in climate and physi cal geography. But perhaps their most impor tant service to the geologist is as factors in the determination of the geological age of the rocks. Their use in this respect depends upon two important principles—one that the strata are normally found in the order of their deposi tion, the oldest below, the highest above. This is known as the law of superposition of strata. The second principle is that, in the evolution of life on the globe, there has been a general upward progression. A knowledge of the nature of this progression, therefore, makes it possible, by a study of the fossils of given strata, to tell in what, stage of life development they lived, and to assign an age to the strata in which they are found. The use of the term age in this connec tion naturally does not mean years. A term like the Devonian period might be considered to repre sent in geological history what the term Bronze Age means when applied to human history. It refers to a stage of life development.
Prior to the enunciation of these principles by William Smith about a century ago, there had been various attempts to classify the strata. ,An early attempt employed the three terms Primary, Secondary, and Alluvial. A later attempt elab orated this time division as follows: Primitive, Transition, Secondary, Tertiary, and Alluvial.
In the classification at present widely in use, the term Tertiary is still employed, and Secondary is occasionally met in the writings of geologists of a few years ago. At one period lithological data were used in classifying the strata, on the assumption that at certain periods widespread conditions permitted the general deposit of rocks with certain Ethological characteristics. Thus there was a Carboniferous period or age of coal; an Old and a New Red Sandstone period; a Cre taceous, or Chalk period; an Oolitic period, etc. Several of these inherited terms are still in use even now that it is known that lithological char acteristics were not universal. With the intro duction of the life record it was found possible to define periods of geological history with more definiteness, often placing their boundaries at un conformities which marked a break in the pres ervation of the life record, thus making a good dividing line. This study has led to the neces sity for the introduction of new names and the abandonment of some of the old ones. Very com monly the new names are geographic—Devonian, from Devonshire, England, and Permian, from Penn, Russia, for example—being adopted from the region where the study necessitating the new name was made. The use of fossils has also made it possible to subdivide the larger divisions of geologic history, and the names thus introduced are usually geographical and of local significance. Thus, those of Texas differ from those of New York, California, India, or England. But the large divisions are of world-wide application. The following table gives the names commonly in use in America for the main divisions: In a given region a broad statement of the stratigraphic geology would start with the oldest rocks, perhaps the Archaean, and continue down to the present. It would treat of the fossils, their characteristics, variations, and associations, and it would include a study of the structure, posi tion, and relations of the rocks themselves. These studies would be applied to an interpretation of the history of the region, both in general and in detail: The evolution of life; the climate and its variations; the relation of sea and land, and their variations in relation; the nature of sedi mentation and the conditions accompanying it; the geographic conditions and the changes in past geography, with causes; periods of volcanic activity and their effects; the growth of moun tains and their reduction; in a word, all the many and complex changes and interactions and interrelations of conditions which have helped to make the geological history. It is such a complicated subject that no adequate abstract is possible in an article of this scope. In fact, stratigraphic geology, being a history of the past, differs for each locality, and can be properly dis cussed only in treatises on geology. Much on stratigraphic geology is, however, given in va rious articles on specific topics. See PALEON TOLOGY; PALEOBOTANY; ARCH2EAN SYSTEM; CAM BRIAN SYSTEM; SILURIAN SYSTEM; etc.