GRENVILLE SERIES OF EASTERN NORTH AMERICA 627 



If the quantitative classification be adopted, this rocl-c would find 

 its place as a Tehamose, a division which includes many granites 

 and liparites, and, so far as its chemical composition is concerned, 

 might be of igneous origin, although few liparites contain so high a 

 percentage of silica. Its mode of occurrence and structure, however, 

 indicate that it is of sedimentary origin and it evidently results from 

 the recrystallization of some arkose-like material derived from the 

 disintegration of granite rocks. 



These sedimentary gneisses (paragneisses) clearly represent 

 deposits of argillaceous, or in some cases arenaceous sediment, often 

 more or less calcareous, which were laid down in the same sea in 

 which the limestones were accumulated. 



In a extreme southern portion of the Bancroft area there is a 

 considerable development of clay stones (often siliceous or calcareous) 

 which probably represent these gneisses in a less altered form, but 

 which probably, at the same time, hold a considerable admixture 

 of volcanic material. 



The amphiholites. — Intimately associated with these sedimentary 

 gneisses and the limestones on one hand, and with the gabbros and 

 diorites on the other, are other rocks which are grouped under the 

 name of amphibolite. While many varieties of these rocks occur 

 in the area, differing considerably from one another in appearance, 

 they have as common characteristics a dark color and a basic com- 

 position. Quartz, which is one of the commonest constituents in 

 the gneisses, is absent, or is present only in very small amount, 

 while hornblende and feldspar, the latter chiefly plagioclase, are the 

 chief constituents of the rock. Pyroxene or biotite often replaces 

 the hornblende in part. In places the sedimentary gneisses fade 

 away into occurrence of amphibolite when traced along the strike. 

 Masses of amphibolite also, as has been mentioned, abound as 

 inclusions throughout the granite of the bathyhths. 



These amphibolites, furthermore, are not peculiar to this area but 

 occur abundantly everywhere in the Laurentian. They have always 

 proved to be one of the chief difficulties in the way of a correct under- 

 standing of the geology of this system, seeing that it has been impos- 

 sible to do more than indulge in conjectures concerning their origin. 

 The same difficultv has been met with in the case of these and allied 



