SALMON — MINERALOGICAL NOTES. 
67 
m. 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 contraiy, 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, 
same 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 which it is often associated. 
An intimate relation exists between the two rocks, but heat only played a very 
minor part in the formation of trap. Its base is anorthose felspar-, 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 differs I think in having been formed in a 
lower temperature. 
* Granite, as a type of a large class of rocks, has ahnost the same mineral 
composition as trachyte, for it contains quai-tz, felspars, and micas ; but the 
occurrence and chai'acteiistics of these minerals are very different. Its quai'tz is 
particularly worthy of remark. In i-ocks of igneous origin this 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 sihcic 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 anoi-those. The consideration of the micas and other minerals of 
this rock are equally instructive in showing its aqueous origin. 
Diorite has a veiy simple mineralogical composition, being essentially formed 
of anorthose and hornblende, vrith, 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 ail the eruptive rocks, been the most enigmatical. 
Its mineralogical characters ai'e 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 aU effects of heat have entirely 
disappeai-ed, and its plasticity can scarcely be attributable to any other causes 
than water and pressure. 
