408 Prof. T. G. Bonney. On the [Nov. 2'<, 



of these have been analysed by Vom Rath in " Zeitsch. Deutsch. Geol. 

 Gesell.," xxvii (1874), p. 296, and from this I quote two examples 



100 00 100-55 



I. S.G. = 2'647. Loss on heating 0'27. From a spbernlitic obsidian ; 

 Antisana. 



II. S.G.=2'620. Loss on heating O'Ol. From a dark andesite, 

 containing among larger constituents hornblende, some augite, and 

 plagioclase. From the crater of Gnagua Pichincha. 



It is evident that this felspar contains more silica and soda, and 

 less lime than normal labradorite, of which in Dana's table of analyses 

 (see " Mineralogy," s.v. Labradorite) the greatest amount of silica is 

 given as 55"8, while the NaoO more often lies below than above 5, and the 

 CaO ranges from 8 to nearly 13, and rather usually exceeds 9 per cent. 

 Still, as the name andesine is by no means universally accepted, and 

 it is extremely difficult to say what constitutes a species in felspar, I 

 have throughout designated these larger felspars in the Andes rocks 

 as labradorite, with which in their optical characteristics they appear 

 to correspond very nearly. 



A comparison of this paper of Professor vom Bath's (which I had 

 unfortunately overlooked when writing my notes on Antisana), with 

 one by Professor Theod. Wolf in the " Neues Jahrbuch " (1874, p. 377) 

 shows that the pitchstones examined by myself came from the same 

 vicinity as that described by Professor vom Rath in the note to which 

 I referred in Part II. Professor Wolf states that quartz-andesite (which 

 it will be remembered does not occur in its normal condition in 

 Mr. Whymper's collection) is found at Achnpallas, on Antisana, and 

 to the south of Riobamba, as well as at the volcano of Mojanda. It 

 forms the whole hill of Achupallas (12,402 feet), and is largely 

 developed on Antisana. He mentions among the localities Urcucuy 

 and Tablarumi, and speaks of a wonderful perlite lava-stream as 

 occurring on Urcucuy, a hill " im Paramo, nicht weit unter dem 

 Antisana-kegel." It is, therefore, evident that the specimens of pitch- 

 stone brought by Mr. Whymper (obtained from a collector) came from 

 this district, since they bear the label Quebrada de Urcucuy. Professor 

 vom Rath gives an analysis of the glassy part of the rock (I), with a 

 separate one of the spherulites (II). 



