ON ITS EXTEBIOR. VOLCANOES. 435 



Volcanoes which are very near to each other, and 

 which are similar in form, construction of framework, 

 and " geotechtonic " relations, have often a very different 

 individual character in the composition and association 

 of their mineral aggregates. On the great cross fissure 

 which, running almost due east and west from sea to 

 sea, intersects a south-east and north-west mountain 

 chain (or more truly an uninterrupted mountainous 

 swelling), the volcanoes are arranged in succession 

 thus: Colima (13,003 feet), Jorullo (4265 feet), Toluca 

 (15,168 feet), Popocatepetl (17,726 feet), and Orizaba 

 (17,374 feet). The volcanoes which stand next to 

 each other are dissimilar in characteristic composition ; 

 the trachytes of the alternate ones are similar. Colima 

 and Popocatepetl consist of oligoclase with augite (there- 

 fore have the Chimborazo or Teneriffe trachyte) ; Toluca 

 and Orizaba consist of oligoclase with hornblende (there- 

 fore have the ^Egina and Kozelnik rock). The late 

 upheaved volcano of Jorullo, which can scarcely be 

 regarded as more than a great "hill of eruption," 

 consists almost solely of basaltic and pitchy, mostly 

 scoriaceous lavas, and seems to be nearer to the Toluca 

 than to the Colima trachyte. 



In these considerations on the diversity of minera- 

 logical constitution between neighbouring volcanoes, 

 we may at once see a censure of the unfortunate at- 

 tempt to introduce, as the name for a kind of trachyte, 

 a designation taken from a chain of mountains, in great 

 part volcanic, extending over a distance of 7200 geo- 

 graphical miles. The name of " Jura-limestone," 

 which I was the first to introduce ( G08 ), is not objection- 

 able, because it is taken from a simple, unmixed kind 



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