19 



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 

 Pre-Cambrian age, in various parts of the world, and 

 they have been the subject of much investigation and 

 widespread 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 

 having had originated: — 



(i) As the basic differentiation products (Ausschei- 

 dungen) from the granite magma. 



(2) As portions of the rock forming the walls or roof 



of the batholith, which had fallen into the granite 

 magma and had partaken of its subsequent 

 movements. 



(3) As fragments of intrusive masses, dykes, stocks, 

 etc. 



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 some- 

 times suggests this; but the movements in the granite 

 have been so great, and the inclusions have been 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. Smith, Jr., 'Report on the Crystalline Rocks of St. Lawrence County, 

 N.Y. State Mus. 49th Ann. Rep. 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. Comm. Geol. Finl. vol. ii. 

 No. 13 (1902), p. 157, considers that the amphibolites, which are characteristic asso- 

 ciates 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, which, if the 

 granite be supposed to represent the original subcrust in a softened or remelted con- 

 dition, cut through this crust, and were connected with basic effusives at the surface: 

 these masses, having been torn to pieces by the subsequent movements of the softened 

 granite, now appearing as scattered fragments. 



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