﻿Yol. 64.] THE LAURENTIAN SYSTEM IN EASTEEN CANADA. 135 



granite-gneiss of the bathjliths, moreover, streaks of grey gneiss 

 are found. It is estimated that, taking the granite-gneiss of the 

 "whole area examined, the amphibolite-inchisions represent aboub 

 10 per cent, of its volume and this grey gneiss another 10 per cent. 



The origin of these amphibolite-in elusions and of the masses of 

 grey gneiss is not only a question of much interest, but one of 

 considerable importance to the true understanding of the geological 

 processes which have been at work in this region. 



As is well known, similar inclusions of dark basic rocks of the 

 nature of amphibolite are found in very many occurrences of granite, 

 especially those of Archaean age, in various other parts of the world, 

 and they have been the subject of much investigation and wide- 

 spread discussion. By many geologists they have been considered 

 to be basic differentiation-products from the acid magma, while 

 others have looked upon them as fragments of foreign rocks caught 

 up by the granite.^ In the region at present under discussion there 

 are three ways in which it would be possible to consider them as 

 liaving originated : — 



(1) As the basic diiferentiation-products (ausscheidu ngen) from the 



granite-magma. 



(2) As portions of the reck forming the walls or roof of the bathyhth, 



which had fallen into the granite-magma and had partaken of its 

 subsequent movements. 



(3) As fragments of intrusive masses, dykes, stocks, etc., which, if the 



granite be supposed to represent the original subcrust in a softened 

 or remelted condition, cut through this crust and were connected 

 with basic eifusives at the surface: these masses, having been torn 

 to pieces by the subsequeiit movements of the softened granite, now 

 appearing as scattered fragments. 



A careful study of all parts of the area has failed to furnish any 

 ■evidence that the first is the true explanation anywhere. There is 

 positive proof that the second is the correct and only explanation of 

 the inclusions in several parts of the area, and it is an explanation 

 not opposed to the facts in any part of the area. The inclusions in 

 some places, more especially in the great northern granite-gneiss 

 areas, may have originated in part as set forth in the third explanation. 

 The form of the inclusions sometimes suggests this ; but the move- 

 ments in the granite have been so great, and the inclusions have 

 l)een so torn to pieces, that it has been impossible to decide whether 

 any of them have been derived from the source indicated under 

 this head. 



^ C. H. Smvth, Jr., 'Report on the Crystalline Eocks of St. Lawrence 

 County ' N. Y. State Mus. 49th Ann. Eep. 1895, vol. ii (1898) p. 490. The black 

 inclusions in the granite-gneisses of the Adirondacks are considered to be 

 broken masses of an older rock caught up by the granite-gneiss when the latter 

 was still in a molten condition. B. Frosterus, ' Bergbyggnaden i Sydostra 

 Finland ' Bull. Coram. Geol. Finl. vol. ii, No. 13 (1902) p. 157, considers that 

 the amphibolites, which are characteristic associates of the granite-gneiss of 

 Southern Finland, are probably for the most part altered dyke-rocks. Some 

 of them still show a gabbro-like structure. 



