10

EXPLANATION OF THE NOMENCLATURE IN THIS PAPER.

In the present unstable condition of geologic nomenclature it is difficult to describe geologic facts in language that will convey the same meaning to all readers, and hence a few words are necessary concerning the terms used in this work. These are as follows: Group, System, Series, Division, Beds, and Horizon, as exemplified in the section on pages xiii and xiv. The term Group is used to indicate a series of formations produced in an interval coincident with a great geologic time, i. e., the rocks of Mesozoic time are called the Mesozoic group.

System is used to indicate the collection of rocks formed during a geologic period, e. g., the Cretaceous.

Series denotes the structural product of a single uninterrupted geological event corresponding to the writer's conception of a formation—a word which, owing to its ambiguity, however, is not here used. Each series of sedimentary rocks reresents a more or less complete ternary succession of strata, i. e., the succession of deposits, first along shore, followed by deepening, and then by shallowing deposits to land again, as laid down by each subsidence and elevation of the ocean's bottom.

Division is used to express a broad subdivision of the conspicuous features of a series, usually founded upon some change in lithologic aspect, and not necessarily denoting minute differentiation or sharp line of demarkation. It may also be used to denote relative position in a series, as Upper, Middle, and Lower, without at all implying lack of continuity between them.

The term Beds denotes a well defined lithologic stratum or collection of strata within a series, possessing common characteristics, as the Exogyra Arietina clay. This term is equivalent to the word formation as employed by many geologists, and is only used tentatively until a more exact word is invented.

Horizon indicates the point of range of occurrence of a species (lithologic or paleontologic) within a stratum. Thus the horizon of Terebratula Wacoensis, Roemer, is at the top of the Washita limestone beds of the Washita division of the Comanche series of the Cretaceous system of the Mesozoic group.

It has not always been, but is fast becoming, a fundamental principle of geologic investigation that definition of strata should always precede their correlation, and hence local geographic names have been given to the strata described in this work wherever possible. Such nomenclature, however, during the reconnoissance stage of investigation may be imperfect, and subject to change.

The nomenclature used here for these subdivisions, though tentative and subject to change at any time, is considered the best that can be arranged at