STRATIGRAPHY.
Correlation.— Practically all the success that has been attained in working out earth his tory in detail has depended on the ability to correlate beds of rock in widely separated areas. Such stratigraphic correlation rests on two highly important principles. The first of these is that in regions where the beds preserve their normal order, undisturbed by folding, the older beds are below and the younger above. The second is the principle of progressive evo lution; that is, in the earliest geologic periods simple forms of life predominated, but as time went on old forms became extinct and new forms evolved, so that of two assemblages of fossils from rocks of different ages certain fornts may be common to both, certain forms found in the older will be absent from the younger, and certain forms found in the younger will not be present in the older.
Admitting these two principles, let it be as sumed that in an unfolded region three forma tions A, B and C rest one on the other, A being the lowest (oldest) and C the highest (young est). In a region several miles away three for mations are also found, the lowest bed of which has the same fossils as B, the middle stratum the fossils of C, while a still higher formation has fossils not found in the first region. This latter assemblage may be called D. It is at once obvious that D is younger than any bed in the first region. This illustrates in its simplest form the principle of stratigraphic correlation by fossils. If in a distant region which is much folded, rocks with the same assemblages of fos sils are found standing vertical, it thus becomes possible to determine which is the older. See FOSSILS ; PALEONTOLOGY ; PALEABOTANY.
Divisions of Geologic Time.—At various times in the earth's history there have been great though slow diastrophic revolutions, re sulting in the formation of mountain ranges, the emergence of vast areas from the sea, and notable climatic changes. These profound modifications of living conditions have made equally profound changes in life forms, bring ing about the extinction of many unfit genera and the development of new genera better suited to the changed conditions. These more
pronounced revolutions in geography and life have been used as the dividing points between the larger time units in geology, known as eras. Less notable changes of similar nature have given rise to periods. Divisions which suit the facts in one locality may not apply elsewhere, and with increased information there has been a constant growth and modification of time nomenclature. The accompanying table of time units is the one used by the United States Geo logical Survey. With minor modifications the scheme is accepted over practically all of the civilized world. A few of the names are rem nants of the older ideas. Tertiary and Quater nary remain from a day when it was believed all geologic time was divisible into four units, Pri mary, Secondary, Tertiary and Quaternary. Carboniferous (coal bearing) and Cretaceous (chalk bearing) are remnants of the idea that rocks could be correlated on the basis of simi lar composition. It is now known, however, that rocks of identical character may have formed during many widely separated periods.
The tendency now is to derive names from re gions where certain units are well exposed. Devonian from Devonshire, Jurassic, from the Jura Mountains and Huronian from Lake Huron are examples.
The rocks of a certain period are spoken of as a system, for example, the Cambrian system, the Cretaceous system. The smallest units to be given separate rank on a map are subdivisions of the system called formations, which usually, though not always, consist practically of one kind of rock, as the Saint Peter sandstone for mation, the Madison limestone formation, and which are usually given the name of a locality where they outcrop prominently. The major facts of history for each period. will be given in a separate article on that period, these facts including the thickness and nature of the sedi ments, whether terrestrial or marine, chemical or mechanical; the extent and importance of unconformities; geography of the period, whether land or .sea, plains or mountains; cli mate, folding and faulting, volcanism, glacia tion and a careful study of the life record. See CENOZOIC ERA; DEVONIAN ; etc.