TRUE VOLCANOES. 439 



tions of a more profound and less prejudiced character led to 

 the recognition of the trachytic albites as oligoclase.* Gus- 



andesine in the syenite of the Yosges (from the Ballon de Servance, 

 and Coravillers, which Delesse has analyzed). Compare G. Rose, in 

 the already often-cited Zeitschrift der Deutschen geologischen Gesell- 

 schaft, bd. i., for the year 1849, s. 369. It is not unimportant to re- 

 mark here that the name andesine, introduced by Abich as that of a 

 simple mineral, appears for the first time in his valuable treatise, en- 

 titled Beitrag zur Zenntniss des Feldspaths (in Poggend., Ann., bd. 1., 

 s. 125, 341; bd. li., s. 519), in the year 1840, which is at least five 

 years after the adoption of the name andesite, instead of being prior 

 to the designation of the mineral from which it is taken, as has been 

 sometimes erroneously supposed. In the formations of Chili, which 

 Darwin so frequently calls andesitic granite and andesitic porphyry, 

 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 green-stone and 

 green-stone porphyry (in PoggendorfT's Ann., bd. xxxiv., s. 1-30) ap- 

 peared 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 definition of which 

 is, not albite with hornblende, but in the Cordilleras of South Amer- 

 ica, oligoclase with augite. The now obsolete account of the desig- 

 nation of andesite, of which I have perhaps treated too circumstan- 

 tially, helps to show, like many other examples in the history of the 

 development of our plrysical knowledge, that erroneous or insufficient- 

 ly grounded conjectures (as, for instance, the tendency to enumerate 

 varieties as species) frequently turn out advantageous to scieuce, by 

 inducing more exact observations. 



* So early as 1840, Abich described oligoclase trachyte from the 

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

 vnd die Zusammcnsetzung der VuUcan-Bildungai, s. 46), and even in 

 1835 Gustav Rose 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 be- 

 lief formerly so generally entertained, that a decided preponderance 

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

 of the feldspar family, such as glassy orthoclase (sanidine), Labradorite, 

 or oligoclase, appears to be very much shaken by a comparison of the 

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

 and third division. In the basalt formation hornblende and augite 

 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 Chimborazo, 

 Pichincha, Purace, and Teneriffe rocks. Olivins, which are so very 

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

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

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

 the whole, very unusual in basalt, and yet some of the basaltic sum- 

 mits of the Bohemian central mountains, first described by Reuss, 

 Freiesleben, and myself, contain plenty of it. The unusual isolation 

 of certain mineral bodies, and the causes of their legitimate specific 

 association, probabl}* depend on many still undiscovered causes of 



