SALMON — MINERALOGICAL NOTES. 



67 



rn. The non-igneous rocks correspond to Lyell's plutonic rocks. 

 Their plasticity was due to the combined action of water and 

 pressure, heat having only played a very secondary part in their for- 

 mation. In them the constituent minerals are devoid of the vitreous 

 aspect peculiar to the igneous rocks ; and their structure is rarely 

 cellular, but, on the contrary, generally very compact. They are not 

 associated with volcanic rocks, and are consequently attributable to 

 an entirely different mode of origin. As types of this class he selects 

 granite, diorite, and serpentine.* 



Conclusions such as these, so strongly opposed to many of our 

 preconceived notions, are not likely to be received in this country 

 with undue favour. M. Delesse on some points surpasses Bischoff, 



Bame elementary composition as dolerite, differing principally in the presence of 

 a certain quantity of water and volatile matters. 



Trap, properly so-called, may pass into basalt, with whicli it is often associated. 

 An intimate relation exists between the two rocks, but heat only played a very 

 minor psirt in the formation of trap. Its base is anorthose felspai', which is 

 generally the only mineral possible to be recognized ; this is always hydrated, 

 often considerably. It is rich in oxide of iron, and often contains spathic car- 

 bonates. Trap forms the limit of the pseudo-igneous rocks ; and although it is 

 intimately connected with basalt, it diliers I think in having been formed in a 

 lower temperature. 



* Granite, as a type of a large class of rocks, has almost the same mineral 

 composition as trachyte, for it contains quartz, felspars, and micas ; but the 

 occurrence and characteristics of these minerals are very different. Its quartz is 

 particularly worthy of remai'k. In rocks of igneous origin tliis mineral is often 

 entirely wanting ; now, in granite, on the contrary, it is very abundant, amount- 

 ing to even a moiety in certain rocks, yet the total quantity of silicic acid present 

 is not greater than in trachyte ; the greater abundance of free quartz being due 

 to the facihties which the mode of origin of granite afforded this mineral of 

 separating itself from the magma. It must be borne in mind also that this 

 quartz is not only always crystalline, but also always hyaline. The study of the 

 felspars of granite is also very instructive. They are orthose and anorthose, 

 opaque, or at most translucent, never vitreous, and always contain a certain 

 quantity of water, usually trifling in the orthose, but varying from 2 to 100 per 

 cent, in the anorthose. The consideration of the micas and other minerals of 

 this rock are equally instructive in showing its aqueous origin. 



Diorite has a very simple mineralogical composition, being essentially formed 

 of anorthose and hornblende, with, at times, protoxide of iron, sphene, ferro- 

 magnesian mica, and, accidentally, garnet. Diorite greatly approaches granite 

 in its mineral composition. Its metamorphism is analogous, and the one may 

 pass insensibly into the other. Diorite may be considered as formed under con- 

 ditions intermediate between those that have produced trap and granite. 



Serpentine has hitherto, of all the eruptive rocks, been the most enigmatical. 

 Its mineralogical characters are so well known, that it is unnecessary to repeat 

 them ; but it is particularly distinguished by its large per centage of water, 

 varying from 13 to 100 per cent. In this rock all effects of heat have entirely 

 disappeared, and its plasticity can scarcely be attributable to any other causes 

 than water and pressure. 



