Mr. Brooke oJi Isomorphistn. 163 



Analysis of a black variety from Cabo de Gato, a green 

 from Zillerthal, and a while from St. Gothard. 



Black. Green. White. 



Silex 4'2 ... 50 ... 28-4 



Lime 9-8 ... 9-75 ... 30-6 



Magnesia 10-9 ... 19-25 ... 18 



Alumina 7*69 ... 0'75 



Oxide of iron .. 22-69 ... 11 

 But the matrix of the Mack contains a considerable portion 

 of alumina, that of the green much magnesia, and the rvkite 

 is found in magnesian carbonate of lime; and hence the dis- 

 agreeing composition of the three varieties may, he says, so 

 far be accounted for. 



That well-known mineral the Fonlainbleau sandstone af- 

 forded an example of a mixture obvious both to the sight and 

 the touch. But the form being that of carbonate of lime, it has 

 been deemed a crystal of that substance mixed with a very 

 large proportion of siliceous matter; and if the quartz had 

 been present in very much smaller grains, or even in ele- 

 mentary particles, there is no reason to conclude that the 

 mixture might not equally have taken place ; for small quan- 

 tities of silex frequently occur as foreign matter in minerals, 

 and are so regarded even by the advocates of isomorphism, 

 and there does not appear any absolute limit to the propor- 

 tion that may be so involved. 



Haiiy appears, therefore, to have been warranted in ascri- 

 bing the differences of composition of amphibole, from dif- 

 ferent localities, to mixtures of foreign matter derived more 

 or less from the bed or matrix in which the crystals were pro- 

 duced. 



The theory, however, which Rose and others have engrafted 

 on the doctrine of isomorphism in its strict sense, is, that the 

 supposed isomorphous elements may become substitutes for^ and 

 replace each other, in any indefinite proportions, xoithout vary- 

 ing the form of the mineral in which the substitution has taken 

 place. 



But the truth of isomorphism in simple binary compounds 

 having been disproved, the very foundation of the theory of 

 isomorphous substitution would appear to be destroyed ; and 

 it will be afterwards seen, that this theory both wants the di- 

 rect support of facts, and leads to contradictory results. 



In the Annals of Philosophy for .June 1826, is a classifica- 

 tion of minerals by Berzelius, founded, as he says, ujjon the 

 property of isomorphous bodies to replace one another in indefi- 

 nite proportions. 



Y 2 According 



