746 



A TREATISE ON METAMORPH1SM. 



is necessaiy in order that recrystallization shall readily occur. If the 

 massive original rocks at the beginning of the mass movements do not 

 contain water sufficient for recrystallization to take place, it does not appear 

 that they are likely under normal conditions to gain sufficient water from 

 an outside source. But where intrusives are introduced on an extensive 

 scale, this source of water may make good the deficiency, so that recrystal- 

 lization may occur. Apparently the amount of water present during 

 recrystallization must be great enough for at least 1 per cent to be left as 

 combined water; for in the deformed igneous rocks containing less than 1 

 per cent of. water granulation is frequent, and the schists and gneisses 

 formed by recrystallization usually contain between 1 and 2 per cent of 

 water. But doubtless in proportion as rocks are deep seated, so that the 

 pressure is great and the temperature high, a 

 decreasing amount of water suffices for recrystal- 

 lization. 



While the minerals of the rocks are steadily 

 dehydrated during the process of recrystalliza- 

 tion, not all of the water driven off escapes; a 

 small part of it is caught within the recrystal- 

 lizing material; and thus are explained the almost 

 infinite number of minute microscopic water in- 

 clusions contained in the schists and gneisses. 

 In some instances the water bubbles are arranged 

 along planes of fracture, in which case they appear 

 in the sections as rows of bubbles which frequently extend continuously 

 across many individuals. (See fig. 19.) The arrangement of continuous 

 rows of water bubbles across many minute individuals has been taken as 

 evidence that the minerals are original, whereas it is plain that the correct 

 interpretation is that the bubbles themselves are probably secondary inclu- 

 sions, as I have fully explained in another place." 



Liquid carbon dioxide is frequently associated with the water inclu- 

 sions. It is believed that in most cases this material is a portion of the 

 carbon dioxide given off by the decomposition of the carbonates during sili- 

 cation. Another and probably larger portion of the carbon dioxide escapes 



a Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 

 1890, p. 218. 



Fig. 19.— Liquid-filled cavities extend- 

 ing across several quartz individuals 

 without change of direction. From 

 Black Hills quartz-schist. 



