236 
THE GEOLOGIST. 
replace the "Ca ; and S"e and Mn sometimes also replace the ^1 in 
small quantities. Meionite, the transparent variety, is essentially the 
same in composition, although in a different order of silicate, by 
wliich it has less 8i, and consequently a higher O.Q. According to 
Bischof, there is no mineral known capable of undergoing more 
numerous and diverse alterations than Wenierite. 
J^S. Epidote, Pisiazite [l.OJ ; Zoisite. Consists essentially of a silicate of 
Al and Ca, the former base being replaced, often largely, in the 
diflFerent varieties by Fe and Sl-n, and the Oa by J'e. Iron-epidote, 
or Pistazite, the most common variety, has Ca (22), I'e (13), and Fe 
(5). Zoidte, or lime-epidote, has Ca (21),¥e (5), and often a variable 
amount of Fe ; it is usually supposed to be distinguished, as a lime- 
epidote, by absence of J'e ; but, according to Nicol, is rather cha- 
racterised by the small amount of Fe replacing Al. Manganese- 
epidote is distinguished by the replacing nf 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 R bases, 
wliich occurs in a minor degree in Wernerite. 
40. Tourmaline, Schorl [1.0 or .833]. This is one of the most complex of 
the siUcated minerals, including, in its difi'erent varieties, in greater 
or less proportions, all the B and R bases named in XIX. as com- 
bining with Si to form rock-mmerals, and containing, 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 
applicable to the different varieties of this mineral ; and the con- 
sistency of its form through changes wliich cannot be accounted for 
by isonioiiiliism 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 considers 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 considei-s as indicative of the substitution of the acids for each 
other. 
The oxygen-ratio in the R and S bases varies very widely. Ram- 
melsberg has been able to arrange the different proportions into five 
groups, according to the nature of this ratio ; and the arrangement is 
» See Dr. Phipson's remarks in July number of The Geologist (vol. i. p. 299) on 
this acid. 
