298 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



magnesia was effected. In this way I endeavored to explain the 

 alteration, in the vicinity of a great intrusive mass of dolerite, 

 of a gray palaeozoic limestone, which contained, besides a little 

 carbonate of magnesia and iron-oxide, a portion of very silicious 

 matter, consisting apparently of comminuted orthoclase and 

 quartz. In place of this, there had been developed in the lime- 

 stone, near its contact with the dolerite, an amorphous greenish 

 basic silicate, which had seemingly resulted from the union of 

 the silica and alumina with the iron-oxide, the magnesia, and a 

 portion of lime. By the crystallization of the products thus 

 generated, it was conceived that minerals like hornblende, 

 garnet, and epidote might be developed in earthy sediments, 

 and many cases of local alteration explained. Inasmuch as the 

 reaction described required the intervention of alkaline solu- 

 tions, rocks from which these were excluded would escape 

 change, although the other conditions might not be wanting. 

 The natural associations of minerals, moreover, led me to sug- 

 gest that alkaline solutions might favor the crystallization of 

 aluminous silicates, and thus convert mechanical sediments into 

 gneisses and mica-schists. The ingenious experiments of Dau- 

 bre"e on the part which solutions of alkaline silicates, at ele- 

 vated temperatures, may play in the formation of crystallized 

 minerals, such as feldspar and pyroxene, were posterior to my 

 early publications on the subject, and fully justified the im- 

 portance which, early in 1857, I attributed to the intervention 

 of alkaline silicates, in the formation of crystalline silicated 

 minerals.* (Ante, pages 6 and 25.) 



[While we may not question the regeneration of fi'M 

 and zeolites (which are but hydrated feldsdars) by the combina- 

 tion of silicates of alumina, like clay, with soluble alkaline or 

 calcareous silicates, it is evident that this process is not the 

 chief nor the primary one ; since the existence of clay sup- 

 poses the previous existence and decay of feldspars. The dep- 

 osition of immense quantities, alike of orthoclase, albite, and 

 oligoclase in veins which are evidently of aqueous origin, shows 

 that conditions have existed in which the elements of these 

 Proc. Royal Soc., May 7, 1857. 



