296 ORIGIN OF CRYSTALLINE ROCKS. [XII t 



Whence, then, come these silicates of magnesia, lime, and 

 iron, which are the sources of the serpentine, chrysolite, pyrox- 

 ene, hornblende, steatite, and chlorite, which abound in these 

 rocks 1 This is the question which I proposed in 1860, when, 

 after discussing the results of my examinations of the tertiary 

 rocks near Paris, containing layers of a hydrous silicate of mag- 

 nesia, related to talc in composition, among unaltered limestones 

 and clays, I remarked that it is evident " such silicates may be 

 formed in basins at the earth's surface, by reactions between 

 magnesian solutions and dissolved silica " ; and, after some dis- 

 cussion, said " further inquiries in this direction may show to 

 what extent certain rocks composed of calcareous and mag- 

 nesian silicates may be directly formed in the moist way."* 

 Subsequently, in a paper on The Origin of some Magnesian 

 and Aluminous Eocks, printed in the Canadian Naturalist for 

 June, 1860,t I repeated these considerations, referring to the 

 well-known fact that silicates of lime, magnesia, and iron-oxide 

 are deposited during the evaporation of natural waters, includ- 

 ing those of alkaline springs and of the Ottawa River. Having 

 described the mode of occurrence of the magnesian silicate, 

 sepiolite, in the Paris basin, and the related quincite, containing 

 some iron-oxide, and disseminated in limestone, I suggested that 

 while steatite has been derived from a compound like sepiolite, 

 the source of serpentine was to be sought in another silicate 

 richer in magnesia ; and, moreover, that chlorite (unless the 

 result of a subsequent reaction between clay and carbonate of 

 magnesia) was directly formed by a process analogous to that 

 which, according to Scheerer, has, in recent times, caused the 

 deposition from waters of neolite, a hydrous alumino-mag- 

 nesian silicate, approaching to chlorite in composition,}: " the 

 type of a reaction which formerly generated beds of chlor 

 the same way as those of sepiolite or talc." Delesse, subse- 

 quently, in 1861, in his essay on Metamorphism, insisted upon 

 the sepiolite or so-called magnesian marls, as probably the 



* American Journal of Science (2), XXIX. 284; also (2), XL, 49. 

 t Ibid. (2), XXXII. 286. 

 t Pogg. Anna!., LXXI. 288. 



