OF VOLCANIC ROCKS. 17 



others resemble enamel and porcelain, or present appearances which are difficult to 

 describe. The occurrence of sphterolites and lithophysfe add to the wariety of their 

 aspect. Another feature peculiar to the rocks of this family consists in a foliated 

 structure, the folia being often thinner than paper, and presenting an endless variety 

 of color and modifications of texture. Pearlite alone does not participate in the pecu- 

 liar form of foliated structure. 



All these varieties again contain, enclosed, all the different minerals before men- 

 tioned, or only a few of them, or they enclose no foreign substances at all. The rocks 

 of this family are chiefiy of purely volcanic origin, but in some instances currents of 

 them appear to have been ejected through crevices in the older volcanic rocks. 



Order Second — Trachyte. 



The name "trachyte" was first used by Hauy, in his academic lectures, to 

 designate the well-known volcanic rocks composing the Drachenfels on the Rhine. 

 But it did not come into general use, until Beudant, the pupil of the former, intro- 

 duced the name into geological literature, by his justly celebrated work "Travels 

 through Hungary," a book which abounds in sagacious observations on the subject of 

 volcanic rocks. Beudant extended, however, the application of the name over a much 

 wider range than his teacher had done. Several years later, in 1835, L. v. Buch 

 introduced into literature the name " andesite," designating by it certain dark- 

 colored rocks, which were then known, especially through the collections of Al. v. 

 Humboldt and Boussingault, to enter largely into the composition of the volcanic por- 

 tions of the South American Andes. These rocks, which form obviously a part of 

 those which Beudant had comprised under the name " trachyte," were supposed to 

 be particularly distinguished by containing a peculiar species of feldspar which Abich 

 (in 1840) called " andesine." When, however, a few years later, this mineral was no 

 longer considered to be a separate mineralogical species, and its name was dropped, 

 that of " andesite " became obsolete with it, notwithstanding its prior origin. Thence- 

 forth, the range of the varieties of volcanic rocks comprised in the term "trachyte" 

 has been even more enlarged than Beudant had proposed in his dissertation. Eruptive 

 rocks, widely differing in nature — in fact nearly all those of tertiary and post-tertiary 

 age, with the exception of basalt — have been united in it. But by no author, proba- 

 bly, was the application of the name as much extended as by Al. von Humboldt. 7 

 On the other side, however, he was the first, by the establishing of numerous sub- 

 divisions, to draw attention upon the necessity of using separate names for more lim- 

 ited ranges of varieties. Recently, B. von Cotta, J. Roth, and others, have tried to 

 demonstrate, how little reason there had been for dropping the term "andesite." 

 They re-introduced it into petrology; but, in drawing the limits between andesite and 

 cognate groups by principles of artificial classification, they used the name in a sense 

 differing to some extent from that in which it has been applied in the following pages. 



' Cosmos, vol. iv. The first four orders of trachyte were proposed by G. Rose, the other two were added by 

 Humboldt, 



(55) 



