Geologists are well acquainted with the fact that during certain portions of geologic time, through a system, or several systems it may be, the rocks for a considerable region may indicate conspicuous uniformity in their geologic history. Thus, the Appalachian basin, as it is called, extending from New York to Alabama, and several hundred miles in width, presents in all essential features great uniformity in the nature of the deposits, in their order, and in the sequence of the faunas for the large part of the Paleozoic time.

When, however, comparison is made of sections in widely separated regions, as those of Nevada and New York, although the general sequence of faunas is similar, the details of the geologic history, as recorded by the stratigraphic series, are entirely distinct.

In the first case, whatever differences are recorded in different parts of the region, may be directly correlated by the . . .

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