456 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



esting ami in some respects the most important mineral found is zoisite, 

 wliich has been repeatedly analyzed and tested. 



All the more important processes of metasoniatic recrystallization can 

 be traced in the altered sandstones, rocks wliose clastic origin could not 

 be doubted for a moment. In many cases one of the first stages in the 

 process is the resolution of the clastic grains into crystalline aggregates 

 from Aviiich new minerals are again built up. Augite, liornblende, and 

 plagioclase luive been observed which had formed in this manner. The 

 feldspars also crystallize along tiny veins in the slides. A frequent occur- 

 rence is the resolution of quartz grains into plagioclase microlites. The 

 reaction begins on the surface of the quartz grains and produces a fringe of 

 twinned feldspar, microlites in positions approximately normal to the surface 

 of the residual kernel. The microlites do not merely abut against the ker- 

 nel, but penetrate it for a sensible distance like closely set pins in a cushion. 

 Zoisite is present in nearly all the altered sandstones It forms in the ag- 

 gregates which result from the clastic grains, and its microlites sometimes 

 pierce quartz grains from the outside. It is abundant in the granular as 

 well as in the prismatic form. This hydrous mineral forms simultaneously 

 with the other products of metasomatic reciystallization, and does not here 

 represent a decomposition process in rocks already recrj-stallized. 



It is only necessary to suppose the processes indicated above carried 

 further to obtain a product in which the clastic character of the rocks would 

 cease to be evident. The altered sandstones thus form uuder the micro- 

 scope, as they do in the field, transitions from the clastic series to the holo- 

 ciystalline rocks. 



The granular metamorphic rocks of the Coast Ranges are separable 

 under the microscope into several groups, but this is not practicable by 

 unaided vision ; indeed, there are manv cases in which specimens which 

 appear to the naked eye to be not greatly altered sandstones prove under 

 the microscope to be holocrystalline rocks, with none of the microstructure 

 of a sandstone. The most important class of the granular rocks is chiefly 

 composed of plagioclase and augite. It sometimes resembles true diabase, 

 and may conveniently be called pseitdodiahase. The pyroxene sometimes 

 assumes the form of diallage. Another class contains anq)hibole instead of 



