POSSIBLE PRE-CAMBRIAN SEDIMENTS 509 



they, B-B, are elsewhere mapped as pre-Cambrian in either direction 

 along the strike. 



The black limestone (number 3) seems to be quite as highly folded 

 and contorted as the older rocks of the section. On the whole, there ap- 

 pears to the writer no reason to believe that there was any great time- 

 break in the deposition of the sedimentary rocks of this section, or that 

 they are not products of a single cycle of deposition. 



The order of deposition of these rocks appears to be as follows, in 

 ascending order: 



1. Black limestone, with black mica-schist as its marginal or shallow- 

 water equivalent. 



2. Quartzite, quartzose, and micaceous dolomites, which were chiefly 

 deposited in the northern part of the section and barely extend to the 

 northern side of the present exposure of the black limestone. 



3. Gray mica-schist covered the entire trough between the pre-exist- 

 ing igneous ridges. The gray schists probably owe their greater 

 thickness toward the northern end of the section to the fact that the 

 Cambro- Silurian transgression came from the north and possibly also to 

 isoclinal folding. 



As it thus seems certain that the Sutton series contains no pre-Cam- 

 brian elastics in the vicinity of Saint Francis river, it is consequently 

 possible, if not probable, that all the clastic rocks of this series, through- 

 out the district, are altered members of the Quebec group, as in this 

 section. 



The volcanics are the oldest rocks of the region, and from their litho- 

 logic resemblance to the pre-Cambrian rocks of Pennsylvania and other 

 parts of the Appalachians they are thought to be of that age. However, 

 there is as yet no direct proof of this. 



The overlying sediments are always much altered. In some cases the 

 alteration would seem to indicate that these sediments are older than the 

 comparatively unaltered rocks of the basins between the metamorphic 

 ridges, but in other cases they can be traced continuously to rocks of 

 undoubted Cambro-Silurian age. While the different degrees of alter- 

 ation may in all cases be due to differences of position or of susceptibility 

 to metamorphism of the various rocks, it is not necessarily the case. Ac- 

 cordingly the question of the existence of pre-Cambrian sediments in the 

 Eastern townships must yet remain an open one.* 



The Serpentines and Diabases 

 A little to the east of the Sutton ridge and parallel to it there is a 



* J. A. Dresser : American Journal of Science, January, 1906. "A study in the 

 metamorphic rocks of the St. Francis valley, Quebec." 



