92 The Paragenetic Relations of Minerals. 



Serpentine is in all cases a product of the alteration of either eruptive 

 or sedimentary rocks. The minerals imbedded in it are usually pseudo- 

 morphous, as, for instance, phastine, which has undoubtedly originated 

 from bronzite. 



III. Eruftive Rocks. In these the absence of quartz and phengite is 

 very remarkable : — 



Basalt, . . . Astrite, hauyne, sanidine, oligoclase (near 



Predazzo, Tyrol), basaltic amphibole, 

 bronzite, augite, zirkon, corundum, oli- 

 vine, ilmenite, magnetic pyrites. 



Phonolite, . . Nepheline, sanidine, basaltic amphibole, 



semeline, ilmenite. 



Compact felsite (the ] Astrite (rare), liebenerite, pegmatolite, 

 most frequent por- > oligoclase, pistacite, quartz, iron pyrites, 

 phyry), . . J gold. 



Pitchstone, . . Astrite, sanidine, quartz. 



Obsidian, . . . Sanidine. 



Pyroxene (augitic) lava Astrite, sodalite,leucite, sanidine, Labrador 

 pyroxene, olivine, hyalosiderite. 



The last two rocks, which have undoubtedly been formed 

 at a very high temperature, are free from either amphibole 

 or quartz. 



Although clay-slate was considered above as a relatively 

 homogeneous rock, it is, with the exception of that which lies 

 above graywacke, undoubtedly in the greater number of in- 

 stances, a very intimately mixed crystalline mass, probably 

 identical in most respects with mica-slate and gneiss, and as 

 such to be included among the mixed rocks. 



There can be no doubt that granite is chiefly eruptive, 

 both on account of its geognostic position and frequent pene- 

 tration of schistose rocks. However, the assumption that it 

 has been in a state of igneous liquidity, is attended with 

 great difficulties, although it is true that felsite may be 

 formed at a very high temperature. The temperatures at 

 which the mineralogical constituents of granite fuse, differ 

 too widely to admit of the supposition that they were formed 

 from a melted mass. Again silica, when heated with basic 

 silicates, readily enters into combination, forming neutral or 

 acid salts ; thus the slags from iron furnaces consist of bi- or 

 tri-silicates, and contain uncombined silica only when there 

 is a great excess in proportion to the bases. The slags of 



