698 A TREATISE ON METAMORPHISM. 



This is beautifully illustrated by the albite-gneiss of Hoosac Mountain, 11 by 

 augen-gneiss of the French Broad River, and at various other localities. 



The argument that orientation of mineral particles in rocks is primarily 

 due to unequal stress in different directions during crystallization applies 

 equally to the cases of the parallel crystallographic orientation of individ- 

 uals which occasionally occur in rocks crystallizing directly from magmas. 

 Feldspar is not infrequently oriented in such rocks, and the phenomena is 

 known in reference to other minerals. 



conclusion. — From the foregoing it is concluded that the development of 

 the schists is to be explained as a process of chemical action induced by 

 mechanical action, resulting in the constant solution and deposition of the 

 material, or its recrystallization, so as to accommodate it to the changing 

 form of the mass. 



ALTERATION'S UNDER MASS-STATIC CONDITIONS. 



It has just been shown that metasomatic recrystallization largely 

 induced by mass-mechanical action is the most important process in the 

 development of the schists. However, recrystallization may and does take 

 place under mass-static conditions, by means of which minerals are 

 produced characteristic of the zone of anamorphism. The cause for 

 recrystallization under mass-static conditions is the demand of the pressure 

 for less volume. If the minerals be recrystallized or made over into those 

 which have a higher average specific gravity, the volume is lessened and 

 work is done. Changes are likely to be important in proportion as the 

 temperature is high and the pressure great. It naturally follows that 

 recrystallization under mass-static conditions is important where the rocks 

 are very deeply buried, or where orogenic movements have occurred, or 

 where igneous rocks have been intruded, and especially where two or all of 

 these are combined. If a rock has recrystallized during mass-mechanical 

 action or igneous intrusion, and therefore has developed into a schist, under 

 succeeding mass-static conditions recrystallization is particularly likely to 

 recur, for the orogenic movement or the intrusive produces a higher 

 temperature in the rocks than would obtain at the same depth under static 

 conditions. 



"Pumpelly, Raphael, Wolff, J. E., and Dale, T. Nelson, Geology of the Green Mountains in 

 Massachusetts: Mon. U. S. Geol. Survey, vol. 23, 1894, pp. 59-63. 



