422 



COSMOS. 



more than forty-six years ago. Travelers, as I have else- 

 where* said, being merely the bearers of the imperfect knowl- 

 edge of their age, and their observations being deficient in 

 many of the leading ideas, that is to say, those discriminat- 

 ing marks which are the fruits of an advancing knowledge, 

 the materials which have been carefully collected and geo- 

 graphically arranged will almost alone maintain an enduring 

 value. 



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

 count of its earliest application to the rocks of Auvergne and 

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

 ing feldspar, especially AVerner's vitreous feldspar. Nose's and 

 Abich's sanidine, is fruitlessly to break asunder that intimate 

 concatenation of volcanic rock which leads to higher geo- 

 logical views. Such a limitation might justify the expres- 

 sion " that in ^tna, so rich in Labradorite, no trachyte oc- 

 curs." Indeed, my own collections are said to prov'e that " no 

 single individual of the countless volcanoes of the Andes con- 

 sists of trachyte ; that, in fact, the substance of which they 

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

 at that time (1835) always erroneously considered to be al- 

 bite, all kinds of volcanic rock should be designated andesite 

 (consisting of albite with a small quantity of hornblende)."! 

 Gustav Rose has taken the same view that I myself adopted, 

 from the impressions which I brought back wdth me from my 

 journeys, on the common nature of all volcanoes, notwith- 

 standing a mineralogical variation in their internal composi- 

 tion ; on the principle developed in his admirable essay on 

 the feldspar groups, J in his classification of the trachytes, he 

 generalizes orthoclase, sanidine, the anorthite of Mount Som- 

 ma, albite, Labradorite, and oligoclase, as forming the feld- 

 spathic ingredient of the volcanic rocks. Brief appellations 

 which are supposed to contain definitions led to many ob- 

 scurities in orology as well as in chemistry. I was myself 

 for a long time inclined to adopt the expressions orthoclase 



* Humboldt, Kleinere Schriften, bd. i., Vorrede, s, iii.-v. 



t Leop. V. Buch, in Poggend., Annalen, bd. xxxvii., 1836, s. 188, 190. 



X Gustav Rose, in Gilbert's Annalen, bd. Ixxiii., 1823, s. 173 ; and 

 Annates de CIdmie et de Physique, t. xxiv,, 1823, p. 16. Oligoclase was 

 first held by Breithaupt as a new mineral species (Poggendoi-fTs Anna- 

 len, bd. viii., 1826, s. 238). It afterward appeared that oligoclase >vas 

 identical with a mineral which Berzelius had observed in a granite 

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

 resemblance in its chemical composition, he had called "Natron Spo- 

 dumen." (Foggcndorff s AnnciL, bd. ix., 1827, s. 281.) 



