452 COSMOS. 



their age, and their observations being deficient in many of 

 the leading ideas, that is to say, those discriminating marks 

 which are the fruits of an advancing knowledge, the mate- 

 rials which have been carefully collected and geographically 

 arranged, will almost alone maintain an enduring value. 



To confine the term trachyte, as is frequently done (on 

 account of its earliest application to the rocks of Auvergne 

 and of the Siebengebirge, near Bonn) to a volcanic rock con- 

 taining felspar, especially Werner's vitreous felspar, Nose's 

 and Abioh's sanidine, is fruitlessly to break asunder that in- 

 timate concatenation of volcanic rock which leads to higher 

 geological views. Such a limitation might justify the ex- 

 pression " that in Etna, so rich in labradorite, no trachyte 

 occurs." Indeed my own collections are said to prove that 

 " no single individual of the countless volcanoes of the Andes 

 consists of trachyte ; that in fact the subtance of which 

 they are composed is albite, and that therefore, as oligoclase 

 was at that time (1835) always erroneously considered to be 

 albite, all kinds of volcanic rock should be designated an- 

 desite (consisting of albite with a small quantity of horn- 

 blende)". 68 Gustav Rose has taken the same view that I my- 

 self adopted, from the impressions which I brought back 

 with me from my journeys, on the common nature of all vol- 

 canoes, notwithstanding a mineralogical variation in their in- 

 ternal composition ; on the principle developed in his admi- 

 rable essay on the felspar groups, 69 in his classification of 

 the trachytes, he generalizes orthoclase, sanidine, the anor- 

 thite of Mount Somma, albite, labradorite and oligoclase, as 

 forming the felspathic ingredient of the volcanic rocks. 

 Brief appellations which are supposed to contain definitions 

 lead to many obscurities in orology as well as in chemistry. I 

 was myself for a long time inclined to adopt the expressions 

 orthoclase-trachytes, or labrador-trachytes, or oligoclase-tra- 



68 Lop. v. Buch in Poggend, Annalen, Bd. xxxvii, 1836, s. 188, 190. 



69 Gustav Rose in Gilbert's Annalen, Bd. Ixxiii, 1823, s. 173, and 

 Annales de Chimie et de Physique, t. xxiv, 1823, p. 16. Oligoclase was 

 first held by Breithaupt as a new mineral species (Poggendorff's Annalen, 

 Bd. viii, 1826, s. 238). It afterwards appeared that oligoclase was iden- 

 tical with a mineral which Berzelius had observed in a granite dyke 

 resting upon gneiss near Stockholm, and which, on account of the re- 

 semblance in ita chemical composition he had called '' Natron Spodu- 

 men." (Poggendorff's Annal. Bd. ix, 1827, s. 281). 



