68 INDEX TO THE STKATIGEAPHY OF NORTH AMERICA. 



neighborhood of areas that are highly metamorphosed, and it may be that those 

 which are considered older may prove to be newer, but as we have no very definite 

 line to divide newer from older, the Canadian Geological Survey would undertake 

 to state how the distinctions should be made. 



Mr. Willis called attention to the classification of the Paleozoic into metamor- 

 phic and nonmetamorphic and suggested that the same distinction might be applied 

 to the Proterozoic of the northern shield. 



After some discussion, in which Prof. Chamberlin urged that metamorphism 

 was but one factor in discriminating older and newer Proterozoic and that the 

 classification should be based on several factors, Prof. Adams stated that throij.ghout 

 northern Canada there are patches of rock that resemble very closely the Kewee- 

 nawan. There is a very strong presumption, which Dr. George Dawson believed, 

 that it is the most extensive of the Proterozoic formations of North America. In 

 other places occur formations which resemble Huronian. There is a distinction of 

 metamorphism which is also very largely a distinction of age. 



The discussion also brought out the fact that in the region northwest of Hudson 

 Bay the Keweenawan facies is predominant; whereas in the Labrador Peninsula, 

 east of Hudson Bay, the resemblances to the Animikie appear to be closer. It 

 appeared therefore that a classification into older and newer Proterozoic must be 

 one which separates the Animikie and Keweenawan from the Huronian. 



As a result of the discussion it was agreed that the Canadian Geological Survey 

 should be responsible for the classification of the Proterozoic in two groups which 

 should in a general way express a relation of older formations to younger ones but 

 which should be clearly explained to be somewhat indefinite and subject to future 

 revision. 



The use of the term Laurentian in general was then considered at length. By 

 reference to figure 4 it wiU be seen that four different interpretations were repre- 

 sented in the committee. The most comprehensive allied Laurentian in general 

 with the Archean of Dana, Laurentian in general being understood to mean the 

 pre-Cambrian rocks of the great northern .shield, regarding which it is not definitely 

 known whether they are prevailingly pre-Hiu-onian or include also large areas of 

 Hutonian: The second interpretation of the Laurentian in general is that which 

 relates it to the Archean according to the restricted definition comprising Keewatin 

 and intrusives into the Keewatin — that is, the pre-Huronian formations to which 

 Logan originally gave the name Laurentian. The third interpretation is that 

 which is represented in the Canadian classification (italics in fig. 4) and which is 

 closely related to the second but leaves the upper limit of the Laurentian in general 

 somewhat indefinite, it being recognized that throughout the Canadian shield there 

 are many intrusives whose age can never be precisely determined on account of the 

 absence of overlying sedimentary rocks. The fourth interpretation is the restricted 

 Laurentian of the international committee on the Lake Superior and Adirondack 

 regions, which limits the term to acidic rocks intrusive into the Keewatin, except as 

 it may be employed, "preferably with an explanatory phrase," to designate some 

 " associated granites of large extent which cut the Huronian or whose relations to 

 the Huronian can not be determined." 



