GEOLOGY OF THE SCHROON LAKE QUADRANGLE 75 



can be made out but not the dip, this being particularly true of 

 the locally gneissoid portions of the Marcy anorthosite. The 

 gneissoid structure is usually accentuated by the roughly parallel 

 arrangement of dark minerals, though locally the granite, compara- 

 tively free from dark minerals, is strikingly gneissoid due to 

 excessive flattening of feldspar and quartz. 



Granulation of minerals, especially feldspar, is common in many 

 localities and often highly developed, the more highly foliated 

 rocks generally being most granulated. 



On the accompanying geologic map there are recorded represen- 

 tative strikes and dips of foliation selected from many field 

 observations. 



The writer considers the foliated igneous rocks to be so-called 

 " primary gneisses " whose gneissoid structure was developed as 

 a sort of magmatic flow-structure under moderate compression 

 rather than by severe lateral (orogenic) pressure brought to bear 

 upon the region after the cooling of the magmas. Briefly stated, 

 the writer's explanation follows. 1 During the processes of intru- 

 sion, which were long continued, the great magmatic masses were 

 under only enough lateral pressure to control the general strike of 

 the uprising magmas with consequent tendency toward parallel 

 arrangement of instrusives and invaded Grenville strata; the folia- 

 tion is essentially a flow-structure produced by magmatic currents 

 under moderate pressure during the intrusions ; the sharp variations 

 of strike on large and small scales, and rapid variations in degree 

 of foliation, are essentially the result of varying magmatic currents 

 under differential pressure, principally during a late stage of magma 

 consolidation ; the almost universal but varied granulation of these 

 rocks was produced mostly by movements in the partially solidified 

 magma, and possibly to some extent by moderate pressure after 

 complete solidification ; and the mineral flattening or elongation 

 was caused by crystallization under differential pressure in the 

 cooling magma. 



It would seem, therefore, that the general absence of foliation 

 from so much of the Marcy anorthosite is best explained as the 

 result of the much more uniform (laccolithic) intrusion of this 

 single great body which is much less involved with Grenville masses, 

 or, in other words, to much less forced differential flowage. 



Figure 8 shows a case of sharp variations in strike within a few 

 feet in the granite of Ledge hill. 



1 The writer has presented a rather full discussion of this subject in 

 Jour. Geol., 24:600-16. 1916. 



