STRUCTURE ANT) DERIVATION OF THE ROCKS. 170 



As to the hornblende schists, the field evidence points to their derivation 

 from basic volcanic rocks. In places this derivation can be traced step by 

 step from the massive rock to the schist ; but for the most part no such 

 transition is observable, and at the base of the Keewatin, in contact 

 with the Laurentian, there is commonly found a formation of hornblende 

 schists of whose origin and development we can only judge by comparison 

 with cases where the history of similar rocks has been thoroughly worked 

 out and established beyond question. Teall,* in Scotland, and Reusch,f in 

 Norway, have shown that some typical hornblende schists and more chloritic 

 hornblende schists may be produced by the shearing of diabase dikes. The 

 writer has collected specimens of the crushed and scpieezed diabase dikes of 

 Bommelo described by Reusch, which are indistinguishable from many of 

 the schists of the Keewatin on the Lake of the Woods and Rainy lake. 

 Teall's description of the hornblende schists resulting from the shearing of 

 dikes would also apply to many of the Keewatin schists which occur in bedded 

 formations. The augite-porphvrites of the Silurian of the southeast coast 

 of Norway, which have been described by Brogger,^ are, at the contact with 

 the intrusion of the augite-syenite of Laugesundf jord, where observed by 

 the writer, altered in places into black glistening hornblende schists, which 

 are very similar to the hornblende schists of the Keewatin at its contact 

 with the Laurentian gueisses. Thus, both the conclusions arrived at in the 

 field and supported by microscopic studies, and the analogies furnished by the 

 investigations of geologists elsewhere, point to the derivation of the bulk of 

 the hornblende schists from normal volcanic massive rocks, which were orig- 

 inally bedded with other stratified rocks, either as flows or as injected sills. 

 ( )ther hornblende schists are probably derived from an analogous alteration 

 of tuffs of basic volcanic rocks. 



The amphibolites are rocks very analogous to the hornblende schists in 

 mineralogical composition, but massive or non-schistose in structure. They 

 have probably undergone the same chemical development as the schists, 

 with pressures so adjusted that no foliation was induced. They are compar- 

 atively local in their occurrence and do not generally make extensive for- 

 mations. 



The various serpentines, so far as they are known, are for the most part 

 beyond doubt the alteration products of local bosses of highly magnesian, 

 massive irruptive rocks. This conclusion is based not simply upon the 

 investigation of the rocks of this particular field by the writer, but upon 

 the numerous instances that might be cited from the petrographical writings 



* Metamorphosis of DoleriCe into Hornblende-Schist; Quart. Jour. Geol. Soc, Vol. XLI, May, 

 1885, p. 133. 



t Bommeloen og Karmoen med omgivelser geologish beskrevne, 1888, pp. 392-307. 



{Spaltenverwerfungenin derGegend Lan^esund: Nyt Magazin for Naturvidenskaberne, XX VIII 

 Hind, 3die— 4de Hefte, p. 352. 



XXIV— Bull. Geol. Soi Am., Vol. 1, 1889 



