TRUE VOLCANOES. 431 



Tuiiguragua, and trachyte rocks, which arc covered by the 



Abich'8 Analysis. 



(Height, 1G,179 English feet; specific gravity, 2-GS5.) 



< txygeo. 



Silicic acid 65*09 ... 83-81 2-G8 



Alumina 15*58 



()w dot* iron 3 - 83 



Protoxyd 1-73 



Lime 2*61 



Magnesia 4-10 



Soda 4-4G 



Potash 1*99 



Chlorine, and loss by heat... 0*4:1 



DO- 80 



In explanation of these figures it must be observed that the first se- 

 ries gives the ingredients in a percentage, the second and third give 

 the oxygen contained in them. The second space shows only the 

 oxygen of the stronger oxyds (those which contain one atom of oxy- 

 gen). In the third space this is recapitulated, so as to offer a compar- 

 ison with that of the alumina earth (which is a weak oxyd) and of the 

 silicic acid. The fourth space gives the proportion of the oxygen of 

 the silicic acid to the oxygen of the aggregate bases, which latter are 

 fixed = l. In the trachyte of Chimborazo this proportion is — 2-33: 1. 



"The differences between the analyses of Rammelsberg and of 

 Abich are certainly important. Both analyzed minerals from Chim- 

 borazo, from the relative heights of 19,194 and 1G,179 feet, which 

 were broken off by you, and were taken from your geological collec- 

 tion in the Royal Mineral Cabinet at Berlin. The mineral from the 

 lower elevation (scarcely 400 feet higher than the summit of Mont 

 Blanc), which Abich has analyzed, possesses a smaller specific gravity, 

 and in correspondence therewith a greater quantity of silicic acid, 

 than the mineral taken from a point 2918 feet higher, analyzed by 

 Rammelsberg. Assuming that the argillaceous earth belongs only to 

 the feldspathic ingredient, we may reckon in the analysis of Rammels- 

 berg : 



Oligoclase 58-GG 



Augite 34-14: 



Silicic acid 4*08 



As thus, by the assumption of oligoclase, a portion of silicic acid re- 

 mains over uncombined, it is probable that the feldspathic ingredient 

 is oligoclase, and not Labradorite. The latter does not occur with un- 

 combined silicic acid, and if we were to suppose Labradorite in the 

 rock, a greater quantity of silicic acid would remain over." 



A careful comparison of several analyses for which I am indebted to 

 the friendship of M. Charles Sainte-Claire Devillc, to whom the valu- 

 able geological collections of our mutual friend Boussingault are ac- 

 cessible for chemical experiment, shows that the quantity of silicic 

 acid contained in the fundamental mass of the trachytic rocks is gen- 

 erally greater than in the feldspars which they contain. The table 

 kindly communicated to me by the compiler himself in the month of 

 June, 1857, contains only five of the great volcanoes of the chain of 

 the Andes : 



