35 



great plains to the south has been an area of progressive 

 sinking, since upon it has been deposited in successive 

 stages a series of great systems of sedimentary rocks. 



Here along the border of these two great geological 

 units, the deep erosion reveals, it would seem, the mechan- 

 ism of elevation, the granite magma rising from the depths 

 and in all probability passing out from beneath the subsid- 

 ing area to the south, lifting the old Laurentian highlands as 

 the liquid in the Bramah press lifts the ram when the 

 piston sinks. 



IX. — Summary. 



(i) The Laurentian system of Sir William Logan in 

 Eastern Canada consists of a very ancient series 

 of sedimentary strata — largely limestones — in- 

 vaded by enormous volumes of granite in the 

 form of batholiths, representing what Logan 

 termed the Fundamental gneiss. 



(2) This sedimentary series is one of the most im- 



portant developments of Pre-Cambrian rocks in 

 North America, and presents the greatest body 

 of Pre-Cambrian limestones on that continent. 



(3) This great Pre-Cambrian limestone series is best 

 designated as the Grenville series. 



(4) The invading batholiths of granite are of enormous 



extent. They possess a more or less distinct 

 gneissic structure, due to the movement of the 

 magma, which developed a fluidal and, in the 

 later stages of intrusion, a protoclastic structure 

 in the rock. 



(5) The granite-gneiss of the batholiths not only 



arched up the invaded strata into a series of 

 domes, but ' stoped ' out portions of the sides and 

 lower surface of the arches, the fragments torn 

 off from the walls and roof by the invading granite 

 being found scattered throughout the mass of the 

 invading rock. This 'stoping' [4, p. 269], how- 

 ever, probably developed only a small part of 

 the space which the granite now occupies. 



This structure is thus identical with that found 



by Dr. Lawson in the Keewatin area of the Lake 



of the Woods district, west of Lake Superior, 



and by Adams in the district north of the 



35064— 3 * 



