82 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



sitic crystal must enlarge in diameter as well as in length, and such appears 

 to he the fact, for the longer crystals ai-e as a rule also the larger ones. 



Zoisite is unknown in eruptive rocks, except as an epigenetic constitu- 

 ent. In tlie Coast Ranges the relations of tlie zoisite to the disintegrating 

 clastic elements of the altered sandstones are such as to forbid any suppo- 

 sition except that it is authigenetic. The granites contain none. 



saussurite. — lu 185!) Dr. T. Stcrry Hunt' showed that tlie saussurite of 

 the euphotide of Monte Rosa corresponded in chemical composition and 

 physical character to zoisite. After the application of the microscope to 

 the study of lithologv, saussurite was recognized as (ordinarily, at all events) 

 a mixture. In 1883 Mr. A. C'athrein' sliowed that many saussurites were 

 mixtures of zoisite and felds})ar. ^lany of the metamorphic rocks of the 

 Coast Ranges might be described as saussuritic, but it appears unadvisable 

 to retain distinct names for mixtures of this description after their real com- 

 position is established. 



In the California rocks this mixture is not a product of decomposition 

 under ordinary conditions, but of a process of recrystallization inconsistent 

 with ordinary decomposition. In Switzerland and elsewhere eruptive dia- 

 basic rocks are supposed in some instances to have been converted into 

 saussuritic masses, while in others decomposition has yielded no zoisite In 

 any case the result of a chemical process must be that group of compounds 

 the formation of which liberates heat most rapidly. It is consequently to 

 be supposed that the saussuritic gabbros have been sul:)jected to influences 

 different from those to which such gabbros as have inidergone ordinary de- 

 composition have been exposed. Judging from the analogy of the rocks 

 of the Coast Ranges, it may be conjectured that the saussuritic gabbros 

 stand to ordinary rocks of the same species in tlie same relation as the 

 zoisitic altered rocks of the Coast Ranges do to those which have merely 

 weathered, or, in other words, that the saussuritic rocks are the result of 

 a process of metamorphism acting upon rocks some of which are eruptive. 



Feldspars. — Thc formatiou of feldspars is an almost invariable accom- 

 paniment of the metasomatic changes of the rocks of the quicksilver belt, 



' Am. Jour. Sci., 2d series, vol. 27, 1650, p. 336. 



•Zoitschr. fiir Kryst. uud Mineral., Grotli, vol.7, 1S83, ji. 234. 



