TRUE VOLCANOES. 467 



imperfectly investigated in a scientifically geological and 

 chemical sense. Modifications in the nomenclature of the 

 associated minerals, as well as additions to the trachyte- 

 formations themselves, are to be expected in two ways, both 

 from the progressive improvement of mineralogy itself (in a 

 more exact specific distinction both with regard to form and 

 chemical composition), and from the increased number of 

 collections, which are for the most part so incomplete and 

 so aimless. Here, as in all other cases where the governing 

 law in cosmical investigations can only be discovered by a 

 widely-extended comparison of individual cases, we must 

 proceed on the principle that everything which, in the 

 present condition of science, we think we know, is but a 

 small portion of what the next century will bring to light. 

 The means of early acquiring this advantage lie in profusion 

 before us, but the investigation of the trachytic portion of 

 the dry surface of the earth, whether raised, depressed, or 

 opened up by fissures, has hitherto been very deficient in the 

 employment of thoroughly exhaustive methods. 



Though similar in form, in the construction of their frame- 

 work and their geotectonic relations, volcanoes situated 

 very near each other have frequently a very different indi- 

 vidual character in regard to the composition and association 

 of their mineral aggregate. On the great transverse fissure 

 which, extending from sea to sea, almost entirely in a direc- 

 tion from west to east, intersects a chain of mountains, or, 

 more properly speaking, an uninterrupted mountainous swell, 

 running from south-east to north-west, the volcanoes occur 

 in the following order :- Colima (13,003 feet), Jorullo (4265 

 feet), Toluca (15168 feet), Popocatepetl (17,726 feet), and 

 Orizaba (17,884 feet). Those situated nearest to each other 

 are dissimilar in the composition which characterizes them, 

 a similarity of trachyte occurring only alternately. Colima 

 and Popocatepetl consist of oligoclase with augite, and conse- 

 quently have the trachyte of Chirnborazo or Teneriife; 

 Toluca and Orizaba consist of oligoclase with hornblende, 

 and consequently have the rock of ^gina and Kozelnik. 

 The recently formed volcano of Jorullo, which is scarcely 

 more than a large eruptive hill, consists almost alone of 

 scoriaceous lavas, resembling basalt and pitchstone, and 

 seems more like the trachte of Toluca than that of Colima. 



