MINKEALOGY; WITH A CLASSIFICATION OF SILICATES. 



71 



elsewhere noted the occurreuce of this material in rock-masses/ but it also appears as a 

 result oi' the alteration of crystals of other species. Finite is essentially a silicate of potash 

 and alumina, haA'iug the atomic ratios of 1 : 8 ; 12 : 3, and thus approaching closely in 

 composition to a hydrous muscovite. Cossaite, which has the ratios of 1 : 9 : 12: 2, is, if 

 not a pinitoid, a compact paragonite or hydroiis soda-muscovite, and giimbellite. which 

 resembles it, with the ratios, 1 : 12 .• 21 : 5, is probably rather a compact phylloid than 

 a pinitoid. A careful study of the optical characters of these species will serve to lix 

 their place iu the system of classification. Jolly te, the most protobasic of the species which 

 we have inchtded in this tribe, has the atomic ratios, 1:2:3:2, the protoxyd bases 

 being ferrous oxyd and magnesia. The minerals known as fahlunite, esmarkite, aspasio- 

 lite and hydrous iolite, are amorphous silicates with A'arying amounts of water and the 

 atomic ratios, 1 : 3 : 5, — the protoxyd bases, as in jollyte, being chielly ferrous oxyd and 

 magnesia, with a little potash. Bravaisite, with the ratios, 1:3:9:4, and hygrophilite, 

 1:5:9:3, are similar species, the protoxyd base of which, as in pinite, is chiefly potash. 



XI. 



Tribe 10. Pinitoid. 



With these pinitoids we have placed obsidian, pitchstone, tachylite and palagonite, to 

 which latter the atomic ratios, 1:2:4, excluding water, have been assigned. That its 

 composition is not clearly fixed, or rather that more than one silicate may have been inclu- 

 ded under this title, does not detract from the interest which attaches to this curious, 

 unstable hydrous colloid, so long since the object of studies by Bunsen, the importance of 

 which I haA'e elsew^here pointed out, and which are noted below in § 109. 



§ 93. The species of the suborder of protopersilicates which approach the persili- 

 cates in composition, resist chemical agencies more than those species which contain 

 larger amounts of protoxyd bases. To this greater stability is dite the fact that such 

 species are often produced by the partial transformation, through aciueous influences, of 

 silicates like the protoperspathoids. Such silicates, formed originally by igneous or by 

 aqueous action, may thus continue to lose protoxyd bases, often with silica, until a condi- 

 tion of comparative fixity is reached by the production of bodies having the chemical 

 composition of pinite, of the muscovitic micas, and even of pyrophyllite or of kaolin. 

 Inasmuch as such compounds are in many cases the result of a secondary process like 

 that just described, chemists have been disposed to assign a similar origin to them wher- 

 ever found, not considering that where the proper chemical conditions unite, these com- 



' The Origin of Crystalline Kocks; Trans. Roy. Soc. Canada, Vul. ii. Sec. :>. p. 52 



