June] HUNT — ON VOLCANIC ACTION. 173 



sedimentary rocks whose history and origin it is, in most cases 

 not difl&cult 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, 

 corresponding to the two groat divisions above noticed. 

 From the mode of their accumulation, however, great 

 variations must exist in the composition of the sediments, 

 corresponding to many of the varieties presented by eruptive 

 rocks. The careful study of stratified rocks of aqueous origin 

 discloses, in addition to these, the existence of deposits of basic 

 silicates of peculiar types. Some of these are in great part 

 magnesian, others consist of compounds like anorthite and 

 labradorite, highly aluminous 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 aluminous silicate, in which lime and magnesia 

 are wanting, and potash is the predominant alkali. In such 

 sediments as these just enumerated 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. As, however, 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 endeavoured to extend and to adapt to the 

 present state of our knowledge. It is proposed in another 

 paper to consider the question of the agencies which have 

 regulated the geographical distribution of volcanic phenomena 

 both in ancient and in modern times. 



Montr&d, Canada, March, 1869. 



* See in this connection the Canadian Journal for 1858, p. 2U3 ; Quart. 

 Geo. Society for 1859, p. 494; Amer. Jour. Science [2] xxxvii., 255, 

 xxxviii. 182; also Geology of Canada, 1863, pp. 643, 669, and Rep. Gool. 

 Canada, 1866, p. 230. 



