28 T. S. Hunt on Voleanie Action. 
Two things become apparent from a study of the chemical 
nature of eruptive rocks ; first, that their composition presents 
such variations as are irreconcilable with the simple origin gen- 
erally assigned to them, and second, that it is similar to that of 
entar rocks, whose istory and origin it is, in most cases, 
not difficult to trace. I have elsewhere pointed out how the 
natural operation of mechanical and chemical ‘agencies tends to 
produce among sediments, a separation into two classes, corres- 
ponding to the two great divisions above noticed. From the 
t Some of these are in great part magnesian, others con- 
sist of bempounds like anorthite and labradorite, highly alum- 
inous basic silicates, in which lime and soda enter, to the almost 
complete exclusion of magnesia and other bases; while in the 
masses of pinite or agalmatolite rock we have a similar ae 
ous silicate, in which lime and magnesia are wanting, and 
ash is t e predominant alkali. In such sediments as these ae 
Spaiiterated we find the representatives of eruptive rocks like 
peridotite, phonolite, leucitophyre, and similar rocks, which are 
so many exceptions in the basic group of Bunsen s, how- 
ever, they are represented in the sediments of the earth’s crust, 
their appearance as exotic rocks, consequent upon a softening 
and extravasation of the more easily liquefiable strata of deeply 
buried formations, is readily and simply explained.* 
The object of the present communication has been to call the 
attention of geologists to the neglected views of Keferstein and 
Herschel, which I have endeavored to extend and to adapt to 
the present state of our knowledge. It is proposed in another 
ay to consider the question of the agencies which have regu- 
pied the Sh Eero mae a of volcanic phenomena both 
ancient and in ti 
_ Montreal, Canada, ia seal 
_ * See in this connection ae Canadian ei ae for Pei p. 203; Quart. Jour. 
Geol. E couse for 1859, p. 494; this Jour., II, xxxv Xxxviii, 182; also 
Sectony of Canada, 1863, pp. 643, 669, po Rep. Geol. ‘unite, 1866, p. 230, 
