Excerpt from Changes in Stratigraphic Nomenclature by the U. S. Geological Survey, 1977 Since 1971 the Subcommission on Precambrian Stratigraphy of the International Geological Sciences has come closer to a consensus on at least the gross subdivision of the Precambrian. James (1978) has presented the recommendations of the 1977 meeting in Cape Town, South Africa, where the subcommission voted to accept, as a first step, a two fold division of the Precambrian into Archean (archaean) and Proterozoic Eons with the boundary defined as m.y. This corresponds to the boundary of Precambrian W and X of the Survey's present scale. The Chief Geologist has approved usage of the terms Archean and Proterozoic. Pending international agreement on subdivision of the Archean, rocks of Archean age will be classified simply Archean and the old designation of Precambrian W is abandoned. Pending international agreement on subdivision of the Proterozoic, the subdivisions X, Y, and Z of the Survey's present Precambrian scale are continued as its subdivisions (see Major Stratigraphic and Time Divisions chart below). The previously used rock stratigraphic and time designations for the Precambrian given in the accompanying listing of nomenclatural changes were used by the authors in reports submitted to the Geologic Names Committee before the new scheme was adopted. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.

Description of three largely volcaniclastic units, which are assigned early Cretaceous age.