234 Dr Charles Baubeny oti (he 



It is observable that the specific gravity of the mineral is, 

 in these cases as well as in most other of the products of ig- 

 neous action, inversely as the amount of silica, and directly 

 as that of the other bases, so that a near approximation to 

 their chemical composition may be often obtained by merely 

 ascertaining their vs^eight. 



This, accordingly, is the method proposed by Abich, in or- 

 der that we may appreciate the real mineralogical composi- 

 tion of a rock, in which the component parts are so blended 

 together that it is impossible to separate one from the other 

 for the purpose of examination. 



In these cases the specific gravity will often give the pro- 

 portion of silica, supposing iron and other of the heavier me- 

 tals not to be present in quantity sufficient to aiFect the re- 

 sult ; and from the proportion of silica the nature of the fel- 

 spathic mineral present may be, in general, estimated with 

 sufficient precision.* 



Now, it is important to observe, that the kinds of felspar 

 commonly found in granite are those which contain the largest 

 proportion of silica, namely, either orthoklase, adularia, or al- 

 bite. Where, as is often the case, orthoklase and albite are 

 both present, the basis is generally composed of the latter, 

 whilst the imbedded crystals consist of the former. 



Such is the case at Carlsbad ; and this fact affords, per- 

 haps, the true solution of a question which I started in my 



Per Cent. 

 * Thus, Trachytic porphyry, having a spe- ) 2.5783 contains of silex 69-46 



cific gravity of J 



Trachyte, 2-6821 65'85 



Domite, 2-6334 65-50 



+ Clinkstone, 2-5770 67-66 



Andesite, 2-7032 64-45 



+ Glassy Andesite, ......... 2-5851 66-45 



Trachyte-dolerite, 2-7812 ... ... 67-66 



Dolerite, 2-8613 63-09 



The only exceptions being clinkstone and glassy andesite, the former having 

 the same composition as trachyte-dolerite, but an inferior specific gravity ; the 

 latter corresponding nearly with clinkstone in both these particulars. It is to 

 be remarked, however, that clinkstone, although chemically resembling trachyte- 

 dolerite, has a different mineral composition, for it appears to be a mixture of 

 a zeolitic mineral with glassy felspar. Probably the same may apply to glassy 

 andesite. 



