COMPARISON OF THE AREAS 287 



The youngest pre-Cambrian rocks are dikes of diabase which are re- 

 ferred to the Keweenawan. 



The feature emphasized by Lawson in his work in this area is that of 

 two widely separated periods of plutonic activity giving rise to batlioliths 

 of granites, to whicli the terms Laurentian and Algoman are applied. 



CoMPARISOlSr OF THE AeEAS 



The problem of correlating areas of pre-Cambrian rocks which are 

 separated by wide intervening stretches of granite is one of the most 

 difficult tasks in geology. At best, all that can be done is to ascertain the 

 most probable correlations from a study of lithology, succession, degree 

 of metamorphism, structure and relationship to intrusives. In Manitoba 

 the most definite feature is the regional granite, which, aside from certain 

 dike rocks, appears to be the youngest pre-Cambrian rock of the province 

 Wherever the relations of the granite to the other pre-Cambrian rocks 

 have been established, the granite has always been shown to be intrusive. 

 This gives us our chief factor in comparing the various areas. It is, of 

 course, possible that the granites of the different areas may be of different 

 ages, or even that granites of more than one age may be represented in 

 the same area, but as yet no proof of this has been obtained. That older 

 granites did exist is shown by the presence of granite boulders in con- 

 glomerates which are intruded by the regional granite. At no place in 

 Manitoba, however, have pre-Cambrian sediments as yet been found rest- 

 ing unconformably on granite. 



The granite locally presents considerable variation. Such variation is, 

 however, to be expected by differentiation and by the assimilation of 

 engulfed fragments of the intruded rocks. Differences due to these causes 

 have been recognized at a number of places. There are also banded 

 gneisses which have the appearance of being older than the fresh-looking 

 granite, but many of these can be shown to be sediments injected "lit- 

 par-lit" by granite. The only proper basis for the determination of the 

 age of an intrusive is its relation to sedimentary formations, and oven 

 this becomes certain only where the sediments can be definitely corre- 

 lated. It is considered, therefore, advisable to avoid the use of such terms 

 as x^lgoman and Laurentian in connection with the granites of Manitoba. 



Though the exact correlation of the pre-granite complexes of sediments 

 and volcanics in A^arious districts or the correlation even of similar litho- 

 logical parts of it seems to be inadvisable, nevertheless the pre-granite 

 rocks in a large way exhibit a great deal of similarity throughout the 

 various areas of the province. The lithology is very uniform. The same 



