I . s C. Schit chert — Pre- Cambrian Nomenclature. 



interval of no deposition, between the sediments resting on the 

 worn surface of the granite and the sediments into which the 

 granite is intrusive, constitutes an unconformity of a major 

 order. "We may for practical purposes take the appearance of 

 a worn surface of granite upon which as a basement sedimen- 

 tary strata rest as prima facie evidence of a major uncon- 

 formity " (12). 



On the basis of these principles and their application by 

 Lawson and many other geologists in fifteen districts in the 

 Great Lakes, Ontario, and Adirondack regions, Lawson pre- 

 sents a " Correlation of the pre-Cambrian on the basis of two 

 and only two granitic invasions." The first column of this 

 table is reprinted here and is set side by side with the one in 

 the Pirsson-Schnchert Text-book of Geology. It will be seen 

 that there are several nomenclatorial differences, and one 

 marked discord regarding the time of the second granitic 

 invasion, the Algomian Revolution. The correct determina- 

 tion of the latter point is not within the writer's scope, and is 

 left to those knowing the field relations. 



The above two tables show that we agree that there were 

 two times of granitic invasions — Laurentian and Algomian — 

 and that there are two major unconformities. The writer 

 believes (not knows) that the Keweenawan and more especially 

 the Animikian are pre-Cambrian in age, that is, are older than 

 the Waucobian or Olenellus fauna, the accepted base of the 

 Paleozoic era. Holding to this belief, it follows that there 

 should be another major unconformity above the Keweenawan 

 and below the "Waucobian of the Cambrian. Therefore the 

 writer divides pre-Cambrian time into three eras, while Lawson 

 holds that there is but one — Archean — and refers the Ani- 

 mikian and Keweenawan doubtfully to the Paleozoic era. 

 What the writer calls eras and periods, Lawson terms periods 

 and epochs. This seemingly trivial matter is, however, not 

 one of nomenclature but is of fundamental importance in the 

 classification of geologic time. In other words, are we to hold 

 with Lawson (1) that all pre-Cambrian time is structurally rep- 

 resentative of but one era; (2) that eras may have within 

 themselves "revolutions," "major unconformities," and very 

 long intervals of erosion ; and (3) since pre-Cambrian time "is 

 blocked out into three grand divisions by the two granitic 

 invasions," that these two " revolutions," as Lawson also terms 

 them, are but of the value of the breaks that separate the 

 accepted periods of post-Keweenawan time ? Long before the 

 writer presented a text-book on Historical Geology most geol- 

 ogists were holding that revolutions and major unconformities 

 were indicative of era delimitation. Further, that pre-Cam- 

 brian time was as long as and even much longer than all Pale- 



