TRUE VOLCANOES. 47 1 



aition of the trachytic albites as oligoclase. 83 Gustav Host. 

 has come to the general conclusion that it is very doubtful 

 whether albite occurs at all among the minerals as a real and 

 essential element of commixture ; consequently, according to 

 the old conception of andesite, this mineral would actually 

 be wanting in the chain of the Andes. 



The mineralogical condition of the trachytes is imperfectly 

 recognised if the porphyritically enclosed crystals cannot be 

 separately examined and measured, in which case, the in- 

 vestigator must have recourse to the numerical proportions 

 of the earths, alkalies and metallic oxides, which the result 

 of the analysis furnishes, as well as to the specific gravity of 



and andesitic porphry rich in albite (Geological Observations on South 

 America, 1846, p. 174), oligoclase may also very likely be obtained. 

 Gustav Rose, whose treatise on the nomenclature of the minerals allied 

 to greenstone and greenstone-porphyry (in Poggendorff's Ann., Bel. 

 xxxiv, s. 1 30) appeared in the same year, 1835, in which Leopold von 

 Buch employed the name of andesite, has not, either in the treatise just 

 mentioned, or in any later work, made use of this term, the true defini- 

 tion of which is, not albite with hornblende, but in the Cordilleras of 

 South America, oligoclase with augae. The now obsolete account of 

 the designation of andesite, of Avhich I have perhaps treated too cir- 

 cumstantially, helps to show, like many other examples in the history 

 of the development of our physical knowledge, that erroneous or 

 insufficiently grounded conjectures (as, for instance, the tendency to 

 enumerate varieties as species) frequently turn out advantageous to 

 science, by inducing more exact observations. 



86 So early as 1840, Abich described oligoclase-trachyte from the 

 summit-rock of the Kasbegk and a part of the Ararat (Ueber die Natur 

 und die Zusammensetzung der Vulkan-B'ddungen, s. 46), and even in 

 1835, Gustav Hose had the foresight to say that though "he had not 

 hitherto in his definitions taken notice of oligoclase and pericline, yet 

 that they probably also occur as ingredients of admixture." The belief 

 formerly so generally entertained that a decided preponderance of 

 augite or of hornblende might be taken to denote a distinct species of 

 the felspar family, such as glassy orthoclase (sanidine), labradorite or 

 oligoclase, appears to be very much ?haken by a comparison of the 

 trachytes of the Chimborazo and Toluca rocks, belonging to the fourth 

 and third division. In the basalt-formatiou, hornblende and agite 

 often occur in equal abundance, which is by no means the case in the 

 trachytes; but I have met with augite crystals, quite isolated, in 

 Toluca rock, and a few hornblende crystals in portions of the Chim- 

 borazo, Pichincha, Purace, and Teneriffe rocks. Olivines, which are so 

 very rarely absent in the basalts, are as great a rarity in trachytes as 

 they are in phonolites ; yet we sometimes find, in certain lava-streams, 

 olivines formed in great abundance by the side of augites. Mica is on the 



