Notes, Reviews and Comments. i09 



which about iooo square miles is anorthosite occurring as a series of 

 great intrusions. 



As a result of the various petrographical examinations undertaken, 

 Dr. Adams has divided the rocks occurring there into four classes. 



i. Anorthosites and granites of igneous origin. 



2. "Augen" gneisses, granulites and foliated anorthosites, gene- 

 tically connected with the last group and largely if not exclusively of 

 igneous origin also. The structure characteristic of this class is the 

 cataclastic or granulated structure formed by the mechanical breaking 

 down of the web of the rock under movements induced by great 

 pressure, which movement produced in the rock a foliation more or less 

 distinct according to their intensity. By " leaf gneisses " are understood 

 very finely foliated gneisses very rich in orthoclase and containing 

 numerous chin leaves of quartz they are usually almost free from iron- 

 magnesia constituents. 



3. A series of crystalline limestones and quartzites together with 

 certain geneisses usually found associated with them and which are 

 probably wholly or in part of sedimentary origin. In these rocks the 

 granulated structure is very subordinate or entirely absent. They are 

 characterized by a v.ry extensive recrystallization with the development 

 of new minerals, they also differ from the rocks of classes 1 and 2 in 

 chemical composition. 



4. Pyroxene gneisses, pyroxene granulites and allied rocks whose 

 origin is as yet doubtful. 



In regard to class 2. there can be no doubt as to their origin as all 

 possible gradations may be seen from the massive variety in which the 

 structure is that of an ordinary plutonic rock to those perfectly foliated 

 were the rock is seen to be in an advanced stage of granulation. 



The quartzites included under the third class referred to as forming 

 part of the Laurentian are entirely crystalline and nothing has been 

 detected which distinctly proves them to be of clastic origin although 

 so eminent an authority as Professor Rosenbusch is quoted as saying 

 that the specimens from one locality present structures which indicate 

 that the rock was originally a sandstone. 



