clxii 



NOTES. 



" The differences between the analyses of Rammelsberg and Abich are, indeed, 

 rather considerable. Both the specimens of Chimborazo-rock analysed, one 

 from an elevation of 19,094 feet, and the other from 16,178 feet, were struck 

 off by yourself, and are from your geognostical collection in the Royal Cabinet of 

 Minerals in Berlin. The rock from the lesser elevation (scarcely 400 feet higher 

 than the summit of Mont Blanc), which was analysed by Abich, has a lower 

 specific gravity, and, in accordance therewith, a greater amount of silicic acid 

 than the rock from the higher elevation which was analysed by Rammelsberg. 

 Assuming the alumina to belong exclusively to the felspathic portion, we may 

 reckon in Rammelsberg's analysis : 



Oligoclase 58'66 



Augite - - - 34-14 



Silicic acid - 4'08 



As with the assumption of oligoclase there thus still remains over some free 

 silicic acid, it is probable that the felspathic ingredient is oligoclase, and not 

 labradorite. This latter does not present itself with free silicic acid; and with 

 the assumption of labradorite in the rock, there would remain over still more 

 silicic acid." 



A careful comparison of many analyses for which I am indebted to the 

 friendship of M. Charles Sainte-Claire Deville, who had the free chemical use of 

 any part of the rich geological collections of our common friend Boussingault 

 shows that the quantity of silicic acid, in the ground-mass of trachytic rocks, is 

 mostly greater thaji in the felspars which they contain. The subjoined table, 

 kindly communicated to me by the author, in the month of June 1857, includes 

 five of the great volcanoes of the Andes. 



