164 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



Conclusions.— It will readily be seen to be a consequence of tlie above 

 facts that granular rockvS having precisely the same composition as porphy- 

 ries cannot have been so highly heated as the latter and that granular 

 rocks as a grouji, unless they differ from the porphyries in chemiciil com- 

 position far more than has hitherto l)een suspected, cannot have been 

 subjected to temperatures on the whole so intense. Differences in texture 

 are in a great proportion of cases certainly due to differences in composi- 

 tion, and, even if one were to find a continuous column of rock porphy- 

 ritic at the upper end and gradually passing over into a granitoid mass at 

 the lower end, the occurrence would not prove that the difference in text- 

 ure was due to difference in pressui-e and rate of cooling, unless the com- 

 position were also proved to be identical (an impossibilitj') or it could be 

 shown that the granular rock had once been a real fluid and not merely a 

 half-fused mass full of solid particles of various kinds. Such instances as 

 the lavas of Clear Lake, the mingled granular and porphyritic diorites of 

 the Comstock, and many exposures of granite show that the homogeneity 

 of any single body of massive rock cannot be taken for granted and that 

 differences of composition lead to differences of texture almost certainly 

 greater than those resulting from the weight and slow conduction of thou- 

 sands of feet of rock.' 



ORIGIN OF THE MASSIVE ROCKS. 



Importance of the subject. — Grauitc Underlies the Coast Ranges and the Si- 

 erra Nevada, and much of the surface of these ranges is flooded with lava. 

 The question of the origin of these rocks is of great importance to a thor- 

 ough discussion of the ore deposits, for it is from the granite or the lava 

 that the ore is most likely to have been derived. The genesis of the re- 



' I have discussed this subject more fully iu a paper on The texture of massive rocks: Am. Jour. 

 Sci., 3d series, vol. 3:5, 1887, p. .'lO. Prof. A. Lagorio has published a very valuable memoir ou the nat- 

 ure of glass base aud on the process of crystallizatiou iu eruptive rocks (Tschermalis mineral. Mii- 

 theil., vol. 8, ltiS7, p. 4;il). This paper reached me after the transmission of this volume. The author 

 carefully considers both the chemical and physical influences affecting the tundency to crystallization. 

 He points out the high alkali contents of the glasses and roaches the conclusion that potassium silicates 

 are I ho last to solidify. He refers granitoid structure to the sudden consolidation under pressure of 

 supersaturated solutions of several salts. This does not seem to me a satisfactory explanation. Simul- 

 taneous supersaturation of a solution of several silicates seems to me improbable, as does also their 

 simultaucous precipitation from supersaturated solution. 



