23G Dr Charles Daubeny on (he 



sistency, possessing a coarse conehoidal fracture, and con- 

 taining a large number of small white crystals, resembling 

 albite, in a crystalline base of a darkish colour. Small cry- 

 stals of glassy felspar are rare in this variety of trachyte, but 

 those of hornblende are common, and augite is also present. 

 It sometimes passes into greenstone or diorite. 



Thus the rock composing the summit of Chimborapo, the 

 basis of which resembles pitchstone, and which is destitute 

 of hornblende, though rich in augite, is called by Von Buch 

 an andesite. Antisana also, and Cotopaxi, are said to consist 

 of the same ; and it is probable that this rock, in connection 

 with trachyte properly so called, constitutes the greater part 

 of those volcanic mountains in South America which are 

 destitute of craters. 



4. Obsidian and pumice, which are so connected, both phy- 

 sically and mineralogically, that they must be placed under 

 the same head, and regarded merely as expressions for two 

 different conditions which the same original material has been 

 made to assume, by the agency of volcanic forces. Both, in- 

 deed, have been regarded rather as particular states which 

 many different minerals are capable of assuming, than as dis- 

 tinct species ; but it is to be remarked, that simple silicates 

 and bisilicates of alumina are incapable of assuming, either a 

 vitreous condition, such as that of obsidian, or those cellular 

 and filamentous forms which we observe in the different 

 varieties of pumice. 



It is necessary therefore that the rock should be rich in 

 silica, or be a trisilicate ; and hence, if with Abich we divide 

 pumices into two groups, namely, into cellular and filament- 

 ous, the former being dark green, poorer in silica, and richer 

 in alumina ; the latter white, and containing more silica ; we 

 shall find that the former is derived from clinkstone, trachyte, 

 and andesite, and the latter from trachytic porphyry.* 



5. Pearlstone, a rock frequent in Hungary, and character- 

 ised by the presence of crystallites, or little globular concre- 



* For further remarks on the formation of obsidian and pumice, see Appendix. 



