40<) 



Prof. T. O. Bonney. On the 



[Nov. _>:. 



slopes." The rock is macroscopically a rather coarse granite, not 

 very well preserved. Microscopic examination shows that there is 

 nothing exceptional in its composition. The felspar is a good deal 

 decomposed, but orthoclase pretty certainly predominates, though a 

 plagioclastic felspar is also present. There is a fair quantity of 

 quartz (containing many cavities, decidedly smaller than those in the 

 tonalite, which are usually empty) together with dark brown mica and 

 green hornblende, and some magnetite. 



Addenda and Conclusion. 



The illustrious traveller Humboldt appears to have brought back 

 specimens from some of the volcanoes visited by Mr. Whymper, and 

 the results of an examination of these by Gnstav Rose are given in 

 "Cosmos" (vol. v, Bonn's edition). The latter refers the rocks of 

 Rncu-Pichincha, Antisana, Cotopaxi, Chimborazo, Tnngaragna, and 

 those beneath the ruins of Old Riobamba to the fourth division of 

 the trachytes, in which " the leading mass contains augite with 

 oligoclase." Humboldt further states that he found hornblende in a 

 separate or sporadic condition " in the rock of Pichincha," also, 

 though not in large quantities, in the " trachytes " of Cotopaxi, 

 Rucu-Pichincha, Antisana, and Tunguragua, along with " augite and 

 oligoclase," but has only recognised it in two of the specimens from 

 Chimborazo. The following table of silica percentages is also given 

 (on the authority of St. Claire Deville) to which I have added those 

 marked thus *. 



t Pitchstone west of Antisana. 



J Of these two rocks one is " from the summit," the other from the crater ; 

 brief microscopical descriptions show that they are hornblende-andesites, with some 

 augite ; substantially identical with some I have described in Part I of these Notes. 



