425 



Immediately connected with the serpentines, there sometimes 

 occurs a white compact rock, remarkable by its great hardness, and 

 a density of 3'3 to 3*5 . Analysis shows this rock to be a pure lime- 

 alumina garnet, in some cases, however, mingled with another sili- 

 cate which appears to belong to the amphiboles. This garnet is 

 sometimes blended with serpentine, and at others forms distinct 

 beds. In its general aspect it resembles closely the saussurite of the 

 associated euphotides, and has probably often been confounded with 

 that mineral by previous observers. Hence the densities of 3' 2 and 

 3*3 assigned by different mineralogists to the saussurites of the Alps, 

 while Delesse has shown that the true saussurite of the euphotide of 

 Mount Genevre, like that of the Vosges, is a felspar. 



The magnesites of this region form great beds ; they are crystal- 

 line, and consist of carbonate of magnesia with some carbonate of 

 iron, and contain as imbedded minerals in some cases grains of quartz, 

 in others felspar and talc, and at other times serpentine, but always 

 holding chrome and nickel, the latter as a greenish carbonate, in the 

 joints of the rock, or in the form of nickeliferous pyrites. 



These magnesian rocks are not confined to the altered portions of 

 this formation ; beds of siliceous dolomite holding protocarbonate of 

 iron are found, interstratified with pure fossiliferous limestones, near 

 Quebec. The reaction between silica and the carbonates of lime, 

 magnesia, and iron, which takes place at no very elevated tempera- 

 ture, in the presence of water, producing silicates of these bases with 

 evolution of carbonic acid, enables us to understand the process 

 which has given rise to the pyroxenes, serpentines, and talcs of this 

 formation, while the argillaceous limestones, which are not wanting, 

 contain all the elements of the garnet-rock. 



The general conclusion deduced from these inquiries, and sustained 

 by a great number of analyses, which I hope soon to submit to the 

 Society, is, that the metamorphism of these Silurian strata has 

 resulted from the chemical reaction, in the presence of water, of the 

 elements existing in the original sedimentary deposits. 



