TRUE VOLCANOES. 473 



cano other easily recognisable, unessential elements of com- 

 mixture, whose presence in large quantities or total absence 

 in different volcanoes, often situated very near to each other, 

 is very striking. Their occurrence, either in frequent abun- 

 dance, 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 originally came, the tem- 

 perature, 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 felspar or 

 from obsidian. These views, which have not been altogether 

 lately adopted, but originated as early as the end of the 1 8th 

 century from a comparison of the trachytes of Hungary and 

 of Teiieriffe, 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 definitions of the mineral 

 species, made by me during my journey have, through Gustav 

 Hose's careful mineralogical elaboration 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 Quelendafia, as also in the subter- 

 ranean pumice-beds of Guapu]o and Zumbalica at the foot 

 of Cotopaxi, 88 but 16 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. 89 In the 

 Continent of Europe micas have shown themselves in abun- 

 d ;nce : 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 



88 Cosmos, see above, p. 343. 



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

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

 canoes 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. 



90 Carl vou Oeynhausen, Erkl der yeogn. Karte des Lacher Sees. 1847, 

 B.3S. 



