18 Schachert and Barrett — Revised Geologic 



Periods of deformation have not found recognition except as 

 events closing periods of sedimentation. Periods of erosion 

 have not been named, and if noted at all in the historic 

 sequence have been represented in tables only by a line of 

 unconformity. In the pre-Cambrian history it is necessary 

 for any adequate representation to give to periods of defor- 

 mation and periods of wide-spread erosion name and place 

 comparable to the periods represented by known sedimentary 

 record. The incorporation of these principles is a feature of 

 this table. Periods of igneous invasion and crustal revolution 

 are shown on the right, periods of sedimentary record on the 

 left. The lesser erosion intervals resulting in the breaks which 

 separate periods are shown by narrow spaces, the great intervals 

 by wide spaces. Even this magnification, however, probably 

 continues to minimize the duration of the great erosion intervals. 

 The arrangement of the table which results serves to make 

 prominent the most significant and distinctive features of the 

 pre-Cambrian, — on the one hand the wide-spread crustal revolu- 

 tions characterized by vast upwellings of molten rocks ; on the 

 other, the profound depth to which erosion has planed, reveal- 

 ing broadly at the surface, levels of the crust once subjected 

 to regional metamorphism at depths measured in miles. In 

 this high average attitude of the lands with respect to the sea, 

 indicated by mountain-building and erosion, the pre-Cambrian 

 resembles the Cenozoic more than it does the Paleozoic and 

 Mesozoic. The completion of the present cycle of erosion 

 will remove wide areas of sedimentary rocks of Mesozoic 

 and Paleozoic age now lying above sea-level, and greatly 

 broaden the exposures of pre-Cambrian rocks. The continental 

 record of the present, like that of the pre-Cambrian, will 

 become one dominantly of diastrophism and erosion, with 

 sedimentation in geosynclines, but all on a lesser scale of 

 magnitude. 



To note in descending order certain of the problems presented 

 in the present table: the attention may be turned first to the 

 use of the names Proterozoic and Archeozoic, with their 

 popular rendition as the Age of Primitive Marine Invertebrates 

 and the Age of Unicellular Life. The reason for this usage is the 

 desire for conformity with the system of classification used for 

 the later geologic ages. The Proterozoic, however, is broken 

 here into an early and a late division, separated by a period of 

 profound diastrophism, following a use made by Coleman in 

 the Dana Memorial Lectures on the Silliman Foundation, given 

 at Yale University in December, 1913, and to be published 

 during 1914 by the Yale University Press. The limitations of 

 these divisions are thus structural rather than biologic, but this 

 is true in a measure also of the, later eras, as argued by 



