TRUE VOLCANOES. 461 



borazo, 80 Tunguragua, and trachyte rocks which are 

 covered by the ruins of Old Riobamba. In the Tunguragiia, 

 besides the augites there occur also separate blackish green 



corresponds to within 32 feet of my own. (Compare my Essay on th 

 If eight of the Mexican Volcano Popocatepetl, in Dr. Petermann's 

 Mittheilungen aus Justus Perthes Geographischer Anstalt, 1856, s. 479 

 481). 



80 In the Chimborazo rock it is not possible, as in the Etna rock, 

 to separate mechanically the felspathic crystals from the ground- 

 mass in which they lie, but the large proportion of silicic acid which 

 it contains, along with the fact connected therewith of the small 

 specific gravity of the rock, make it apparent that the felspathic 

 constituent is oligoclase. The quantity of silicic acid which a mineral 

 contains and its specific gravity are generally in an inverse ratio ; 

 in oligoclase and labradorite the former is 64 and 53 per cent, 

 while the latter is 2.66 and 2.71. Anorthite, with only 44 per cent, of 

 silicic acid, has the great specific gravity of 2.76. This inverse pro- 

 portion between the quantity of silicic acid and the specific gravity 

 does not occur, as Gustav Rose remarks, in the felspathic minerals, 

 which are also isomorphous, but with a different crystalline form. 

 Thus felspar and leucite, for instance, have the same component 

 parts, potash, alumina, and silicic acid. The felspar, however, con- 

 tains 65 and the leucite only 56 per cent, of silicic acid, yet the 

 former has a higher specific gravity, namely, 2.56, than the latter, 

 whose specific gravity is only 2.48. 



Being desirous in the spring of 1854 to obtain a fresh analysis of 

 the trachyte of Chimborazo, Professor Rammelsberg kindly undertook 

 the task, and performed it with his usual accuracy. I here give the 

 results of this analysis, as they were communicated to me by Gustav 

 Hose, in a letter in the month of June, 1854. He says : " The Chim 

 borazo rock, submitted to a careful analysis by Professor Rammels- 

 berg, was broken from a specimen belonging to your collection, which 

 you had brought home from the narrow rocky ridge at a height of 

 more than 19,000 feet above the sea." 



Rammelsberys Analysis. 

 (Height 19,194 English feet; spec. grav. 2.806.) 



Oxygen. 



Silicic acid 59.12 ... 30.70 2.33 



Alumina 13.48 ... 6.30 



Protoxide of irou 7.27 



Lime (5.50 



Magnesia 5.41 2.13} T.93 



Soda ...... 3.46 



Potash 2.64 



97.83 



V.fU ^.O< 



6.30 j ... 

 6.93 J 



