La vis — On the Structure of Rocks. 153 



Abioh. Now, as consolidation takes place, great excesses of 

 either, especially the feebler ones, such as magnetite, are com- 

 pelled to separate ; and as the rock completes its crystallization, 

 the excesses of either form the last crystals, unless the rock 

 suddenly cools before all the vitreous matter has been converted 

 into formed material. Thus, in the acid rocks we have quartz, and 

 in the basic ones magnetite, being the last formed minerals, 

 although the two most infusible of rock-forming minerals, which 

 alone is sufficient to demonstrate that fusion-point has little or 

 nothing to do with the order of separation of the minerals. We 

 should therefore be more justified in determining whether a rock 

 should be regarded as acid or basic by its microscopical structure, 

 than by adopting 60 per cent, of silica as rigidly dividing the two, 

 since the different bases vary much in alkalinity, and oombining 

 proportions, and a magma containing 60 per cent, of silica, might 

 give an acid or an alkaline reaction, according to the quantities of 

 different bases it contained. 



Limit of space prevent further consideration of the different 

 mineral species which go to make up igneous rocks ; the above, being 

 most common, are sufficient to indicate the line of argument 

 followed out. Before, however, quitting the subject, there is one 

 more point worthy of our consideration in relation to the separation 

 of mineral species from a solvent. Different species have been 

 easily obtained from fusion of their components in a saline sub- 

 stance, such as a chloride or sulphate. Thus, for instance, M. 

 Lechartier 1 obtained pyroxene in crystals, a centimeter long, by 

 fusion for a couple of hours in calcium chloride, or sodium sulphate. 

 In the same way wollastonite, apatite, 2 and many other minerals 

 have been obtained by E. Belmen as very perfect crystals from 

 solution in fused chlorides, and other salts, such as vanadates. 

 These facts go to confirm what has been said about the solution of 

 the more infusible silicates in the more fusible ones, and at the 

 same time may account for the occurrence of some minerals that 

 are eruptive, or post-eruptive, in time of their formation. The 

 large amount of sulphates, but especially chlorides, that are vapor- 



1 Comptes rendus, 1868, vol. lxvii., p. 41. 



3 L. Bourgeois, Eacycl. Chim., vol. ii., I"' Appenclice. Reprod. Artif. des Roches, 

 p. 10. 



