Mamesian and Aluminous Rocks, 181 



<s 



acids, whicli talc is not. We cannot however doubt tliat talc 

 and steatite have been formed from sepiolite, which has under- 

 gone a chemical change and become insoluble. It is possible that 

 serpentine may be derived from another silicate richer in mag- 

 nesia than sepiolite. The frequent association of carbonates of lime 

 and magnesia with talc, and of carbonate of magnesia, talc and 

 serpentine, as in the ophiolite of Roxbury, would seem opposed to 

 the notion that serpentine may have been formed from the altera- 

 tion of a mixture of sepiolite and carbonate of magnesia. In 

 chlorite, which often forms rock masses almost without admix- 

 ture, we have an alumino-magnesian silicate which cannot have 

 been derived from sepiolite, inasmuch as this contains for the 

 amount of magnesia present, twice as much silica as chlorite. The 

 oxygen ratios of the silica and magnesia in sepiolite are as 3 : 1, 

 and those of silica, alumina and magnesia (including the vari- 

 able amount of ferrous oxyd which in part replaces the latter) in 

 chlorite are as 6 : 3 : 5, while in the purest clays the ratio of silica 

 and alumina equals 1 : 1, and in most argillaceous sediments the 

 proportion of silica is still greater. It is evident, therefore, that 

 chlorite could not be formed from a mixture of sepiolite w^ith clay, 

 or even with pure alumina, without the elimination of a large 

 amount of silica, and we are led to regard it as having been gen- 

 erated by the reaction of a silicate of alumina or clay with mag- 

 nesia, which was probably present in the unaltered sediment in 

 the form of carbonate. Unless indeed the process, which accord- 

 ing to Scheerer, has in recent times caused the deposition from 

 waters, of neolite, a hydrous alumino-magnesian silicate approach- 

 ing to chlorite in composition, be the type of a reaction which 

 formerly generated beds of chlorite, in the same way as those of 

 sepiolite or talc. 



A silicate of lime allied to sepiolite, has not so far as I am 

 aware, yet been noticed among unaltered sediments, and among 

 crvstalline strata calcareous are more rare than mao-nesian silicates, 

 although double silicates of lime and magnesia (pyroxene and 

 hornblende,) often form beds, and wollastonite, either alone or 

 mingled with carbonate of lime, sometimes constitute rock masses. 

 The double silicates of alumina and lime are however abundant ; 

 the lime-feldspars, scapolite, epidote (saussurite), and white garnet, 

 all form beds in crystalline rocks. Reactions in water at the earth's 

 surface, and at no very elevated temperature, may have given 

 rise to double silicates of lime and alumina corresponding to 



