TRUE VOLCANOES. 441 



which arc referred to in our classification of the trachytes, 

 and which especially characterize them, there exist likewise 

 in each volcano other easily recognizable, unessential ele- 

 ments of commixture, whose presence in large quantities or 

 total absence in different volcanoes, often situated very near 

 to each other, is very striking. Their occurrence, cither in 

 frequent abundance, or else at long and separate intervals, 

 depends probably, in one and the same natural laboratory, on 

 various conditions of the depth from which the matter origin- 

 ally came, the temperature, the pressure, the fluidity, or the 

 quicker or slower process of cooling. The fact of the specific 

 occurrence or the absence of certain ingredients is opposed to 

 certain theories, such as the derivation of pumice from glassy 

 feldspar or from obsidian. These views, which have not been 

 altogether lately adopted, but originated as early as the end 

 of the 18th century from a comparison of the trachytes of 

 Hungary and of TenerifFe, engaged my attention for several 

 years in Mexico and the Cordilleras, as my journals will 

 testify. From the great advancement which lithology has 

 undeniably made in modern times, the more imperfect defini- 

 tions of the mineral species made by me during my journey 

 have, through Gustav Rose's careful mineralogical elabora- 

 tion of my collections, been improved and accurately certified. 



Mica. 



Black or dark-green magnesian mica is very abundant in 

 the trachytes of the Cotopaxi, at an elevation of 14,470 feet 

 between Suniguaicu and Quelendana, as also in the subterra- 

 nean pumice-beds of Guapulo and Zumbalica at the foot of 

 Cotopaxi,* but sixteen miles distant from the same. The 

 trachytes of the volcano of Toluca are likewise rich in mag- 

 nesian mica, which is wanting in the Chimborazo.f In the 

 Continent of Europe micas have shown themselves in abund- 

 ance : at Vesuvius (for example, in the eruptions of 1821- 

 1823, according to Monticelli and Covelli); in the Eifel, in 

 the old volcanic bombs of the Lacher Lake ;| in the basalt 



* Cosmos, see above, p. 323. 



t It is almost superfluous to mention that the term wanting signifies 

 only that, in the investigation of a not inconsiderable portion of volca- 

 noes of large extent, a particular sort of mineral has hitherto been 

 vainly sought for. I wish to distinguish between what is wanting (not 

 being found), being of very rare admixture, and what, though more 

 abundant, is still not normally characteristic. 



X Carl von Oevnhausen, Erkl. der geogn. Karte des Lacher Sees, 

 1847, s. 38. 



T2 



