34 <i RISEN MOUNTAINS IN MASSACHUSETTS. 



have been subjected to the action of great lateral pressure, throwing them 

 into folds and along certain lines into compressed and ruptured overfolds, 

 subjecting the constituent particles to crushing or shearing and to move- 

 ments which are now marked by the crinkling of the original stratification 

 lamination, and by the predominant cleavage resulting from movement. 



There were therefore present the three factors of load, temperature, 

 and attrition of particle on particle produced during the folding movement. 

 These factors were essential in the process of metamorphism, but they could 

 not change ordinary clay sediments into schists consisting largely of mag- 

 nesia and potash micas and abounding in soda-feldspars, nor could they 

 change a grit of quartz and microcline detritus into a gneiss consisting largely 

 of soda-feldspar. Either the original sediments must have contained all of 

 the elements required to form by recrystallization the present constituent 

 minerals, or a part must have been contributed from elsewhere. The 

 extreme rarity of observed eruptive dikes and of pegmatite veins outside 

 of limited areas makes it hard to explain the difference between the chemical 

 constitution of the schists in their great breadth and thickness and that of 

 ordinary argillaceous sediments by ascension from below. It would theie- 

 fore appear more likely that the original sediments were of an exceptional 

 character. They may have been deposited under conditions favorable to 

 the preservation of magnesium and alkaline salts — conditions which we know 

 have at various times existed over large areas. 



In the case of the Lower Cambrian grit the action of mineralizing 

 processes originating below is more probable. Where the rocks have been 

 subjected to the different forms of readjustment of particles during the great 

 folding of the strata, a change occurs from a grit containing much detrital 

 microcline to a highly crystalline gneiss with a predominant soda feldspar, 

 which bears evidence of being crystallized in situ. Along these zones 

 we find veins and "flames" of pegmatite, and in the crushed quartzite proper 

 perfect little crystals of tourmaline often appear in great abundance. The 

 very feldspathic veins along these zones of extreme folding in the grit may 

 stand related causally to the lenses of quartz and tourmaline, with and 

 without feldspar, which occur rather frequently in the higher schists along 

 the west Hank of Hoosac mountain; also along the zone of extreme folding. 



