PRE-CAMBRIAN. 



67 



Bureau of Mines; and T. C. Chamberlin, C. R. Van Hise, and Bailey Willis (chair- 

 man), for the United States Survey. The discussion of the problem of pre-Cambrian 

 classification was earnest and prolonged. It clearly developed the fact that knowl- 

 edge of the pre-Cambrian terranes is not yet adequate to furnish criteria by which 

 even some of the major divisions may be surely correlated or to offset the differences 

 of opinion which spring from historical associations or personal experience. The 

 discussion was in part as follows : 



Mr. Willis presented as a basis for discussion a tentative classification of the 

 pre-Cambrian, which is shown in figure 4 by the names printed in Roman type. It 

 embodied Archeozoic and Proterozoic without definite delimitation; undifferen- 



ES < > 



2 



.23 *♦< 



^ 



o J 



c 



■a 

 I 



'1 



Akn 



Keweenawan 



Ahu 



Upper Huronian 



Ahl 



riiddle and Lower 

 Huronian 



< 



/Rk 



Keewatin 



Hastings 



and 

 Grenville 



KIO 



0;^ 



I 



Eruptive contact 



sy 



/R\ 



Belt and allied 

 terranes 



>S 



>i 



Laurentian 



U' 4 



■2(S 



k 

 ■9 c 



To 



en >> 



■-rt O 

 w C 



c o 

 PS 



Figure 4. — Classification of the pre-C.ambrian adopted by the international committee. The portion in roman type 

 represents the table as originally presented by Mr. Willis. The corrections made by the committee are shown, the 

 additions being in italics. 



tiated pre-Cambrian, which was also designated Laurentian of Canada in general 

 and Archean of Dana; Archean and Algonkian of the United States Geologicial 

 Survey; the classification adopted by the international committee for the Lake 

 Superior district; the Hastings and Grenville series of eastern Canada; the Belt 

 series and allied terranes; and the unclassified schists and intrusives of the Cordillera. 



Prof. Van Hise took up the discussion and emphasized the point that it is 

 desirable in such a map as that under discussion to make clear the relation of the 

 adopted color scheme to the various classifications in use by different organizations 

 and in different textbooks, in order that the matter might b°e plainly understood. 



Prof. Adams, speaking for the Canadian geologists, stated that if the require- 

 ments of cartography limited the classification to three divisions, they would wish 

 one to indicate Laurentian and two others to represent two different parts of the 

 Proterozoic. The divisions of the Proterozoic have to be made on grounds which 

 seem valid now, but they may have to be shifted as the work goes on. The map 

 represents the distinctions essentially as the Canadian geologists would like to 

 have them made, even though in the course of eight or ten years the colors will 

 have to be revised. Some great areas that are little metamorphosed lie in the 



