606 WILLIAM J. MILLER 



very distinctly developed, especially around some of the inclusions 

 in syenite or granite. 



Differences in degree of foliation. — Another important considera- 

 tion is the frequent pronounced variation in degree of foliation in 

 the rocks of the syenite-granite series. They are mostly distinctly 

 gneissoid, rarelv so much so as to be almost schistose, while in other 

 cases they are so faintly gneissoid as to be practically massive. A 

 striking feature is the frequent rapid change within a few rods or 

 yards, from rocks which are very clearly gneissoid to others in 

 which the foliation is scarcely discernible. Sometimes, within a 

 foot or two. a very gneissoid zone lies between others which are 

 only moderately foliated. In many cases there is no evidence 

 whatever of shearing to account for these variations. It seems 

 impossible to conceive that such abrupt foliation changes could 

 ever have been produced by severe compression of the rocks after 

 solidification. Such compression would certainly have brought 

 about a much more uniform degree of foliation. 



According to the writer's view, these variations are best ex- 

 plained as due to forced differential tlowage in the pasty magmas. 

 probably after partial consolidation. Regarding the origin of 

 igneous rock foliation, Pirsson says: ''Sometimes this texture has 

 been imposed upon the igneous rocks after they had solidified, by 

 intense pressure and shearing, and sometimes while they were still 

 soft, pasty, and crystallizing, by forced differential flowage, due 

 to various causes." 1 Those portions of the magma which were 

 forced in probably a more fluid condition between other, probably 

 more past)' or solidified, portions would have had a more perfectly 

 developed foliated structure. 



According to Leith: "Many more schists than gneisses have 



been proved to be the result of mashing of igneous rocks In 



fact, so commonly do the igneous rocks appear when mashed to 

 take on schistose as contrasted with gneissic structure as to raise 

 the question whether gneisses are not exceptional results, most 

 gneisses to be explained as igneous rocks with original flow struc- 

 tures."-' The evidence from the Adirondacks is in harmony with 



1 L. Y. Firsson, Rocks and Rock Minerals (iqoS"), p. 356. 

 - C. K. Leith, Structural Geology (1913), p. 103. 



