236 



THE GEOLOGIST. 



replace the *Ca ; and "Fe and Mu sometimes also replace the ^1 in 

 small quantities. Meionite, the transparent variety, is essentially tlie 

 same in composition, although in a different order of silicate, by 

 which it has less Si, and consequently a higher O.Q. According to 

 Bischof, there is no mineral known capable of undergoing more 

 numerous and diverse alterations than Wernerite. 



39. Epidote, Pistazite [l.OJ ; Zoisite. Consists essentially of a sihcate of 



'M and Ca, the former base being replaced, often largely, in the 

 different varieties by Fe and Mn, and the Ca by te. Iron-epidote, 

 or Pistazite, the most common variety, has Ca (22), Fe (13), and Fe 

 (5). Zoisite, or lime-epidote, has Ca (21), Fe (5), and often a variable 

 amoimt of Fe ; it is usually supposed to be distinguished, as a lime- 

 epidote, by absence of J'e ; but, accordmg to Nicol, is rather cha- 

 racterised by the small amount of "Fe replacing Al. Manganese- 

 epidote is distinguished by the replacing of the Al by Mn (17) with 

 Ca (20), Fe (9). This mineral, it will be seen, is especially marked 

 by the frequent and large interchange of the isomorphous K bases, 

 which occurs in a mmor degree in Wernerite. 



40. Tourmaline, ScJwrl [1.0 or .833]. This is one of the most complex of 



the sihcated minerals, including, in its difierent varieties, in greater 

 or less proportions, all the E and ll bases named in XIX. as com- 

 binmg with Si to form rock-minerals, and containmg, besides, B 

 (which is supposed to act as an acid and replace Si), and also a 

 certain quantity of Fl. No general formula has yet been found 

 apphcable to the different varieties of this mineral ; and the con- 

 sistency of its form thi'ough changes which cannot be accounted for 

 by isomoi-phism has suggested many difficulties. The O.Q. is 1.0 or 

 .833, according as we regard Boracic acid as B or B * ; but, at which- 

 ever number we take it, it only varies, through all the varieties, 

 within very narrow limits, and consequently the ratio of the oxygen 

 in the bases, as a whole, is nearly constant with that of both the 

 acids taken together. Rammelsberg also consider the oxygen in 

 B" to have the constant ratio to that in Si of 1 : 3.5 ; and, however 

 the amount of Si varied, he found this proportion unchanged, which 

 he considers as indicative of the substitution of the acids for each 

 other. 



The oxygen-ratio in the R and R bases varies veiy widely. Ram- 

 melsberg has been able to arrange the different proportions into five 

 groups, accordmg to the natm*e of this ratio ; and the aiTangement is 



* See Dr. Pliipson's remarks in July number of The Geologist (vol. i. p. 299) on 

 this acid. 



