PRE-CAMBEIAN. 65 



Pre-Cambrian intrusive rocks (24). 



1 14 Oklahoma. 



1 16-17 Georgia, North Carolina,. South Carolina. 



J 17-18 : North Carolina, Virginia, Maryland, Delaware, Pennsylvania. 



K 18 Pennsylvania, New York. 



L 17-19 Ontario, Quebec. 



M 15 Ontario. 



M 18-20 Quebec. 



N 20-21 Quebec, Northwest Territories. 



N-0 17-18, P 17-19 Northwest Territories. 



0-P 22-24 Southern Greenland. 



R 22 West coast of Greenland. 



PRE-CAMBRIAN CLASSIFICATION. 



The problem of pre-Cambrian classification has been difficult of adjustment 

 under the requirements of the map of North America. The Canadian Survey adheres 

 to its long-established divisions Laurentian and Huronian, which rest upon the work 

 of Logan and his successors. The modifications developed by modern studies have 

 not in the opinion of Canadian authorities rendered these terms less useful or justi- 

 fied substitutes or material changes of definition. The United States Survey adopted 

 the divisions Archean and Algonkian in 1889 and they have been earnestly advocated 

 by the geologists working in the Lake Superior region in their earlier as well as in 

 their latest publications. These geologists maintain that the terms Archean and 

 Algonkian stand for two great major divisions which constitute a "dual classifica- 

 tion" of the pre-Cambrian that is thought probably to be of general occurrence and 

 to distinguish two grand eras of the earth's history. The points of view of the 

 Canadian and United States geologists have been presented by F. D. Adams ^'^ and 

 C. R. Van Hise *^*'' ^^^ and are also set forth in the report of the special (international) 

 committee for the Lake Superior region.^ 



The classification of the pre-Cambrian which the compiler introduced in the 

 map of 1906 was the result of an attempt to adjust differences of interpretation to 

 the requirements of a general map, in which the number of distinctions was limited. 

 In conformity with the terminology of the international congress the prefixes neo 

 and eo were used for two major divisions of the post- Archean, both of which, accord- 

 ing to the nomenclature employed by the United States Geological Survey, belonged 

 to the Algonkian. Thus arose neo- Algonkian and eo- Algonkian. The classification 

 could not, however, be carried through with perfect separation of the several sys- 

 tems — Archean, eo- Algonkian, and neo- Algonkian — and it became necessary to state 

 that each of these systems as represented on the map of 1906 might include masses 

 belonging to one of the other two. The wording of the explanation in the legend 

 of the map did not make it plain that the Keewatin was included under eo- Algonkian, 

 for instance, only because of the requirements of cartography and in spite of the 

 well-known fact that the two are separated by a great unconformity, and the classi- 

 fication was therefore criticized by American as well as by Canadian geologists. The 

 latter objected to the term Algonkian; the former , objected to the confusion of 

 distinct systems in one. 



48011°— 12 5 



