VI] DISTRIBUTION OF VOLCANOES. 67 



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, cor- 

 responding to the two great 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 care- 

 ful study of stratified rocks of aqueous origin discloses, in ad- 

 dition to these, the existence of deposits of basic silicates of 

 peculiar types. Some of these are in great part rnagnesian, 

 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 sedi- 

 ments as these just enumerated we find the representatives of 

 eruptive rocks like peridotite, phonolite, leucitophyre, and simi- 

 lar 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 ex- 

 plained. 



APPENDIX. 



DISTRIBUTION OF VOLCANOES. 



WE regard the extravasation of igneous matter, whether as lava 

 or ashes at the surface, or as plutonic rock in the midst of strata, as, 

 in its wider sense, a manifestation of vulcanicity ; and, for the elu- 

 cidation of our subject, consider both those regions characterized by 

 great outbursts of plutonic rock in former geologic periods, and 

 those now the seats of volcanic activity, which, in these cases, can 

 generally be traced back some distance into the tertiary age. To 

 begin with the latter, the first and most important is the great con- 



