264 COSMOS. 



position of the different rocks in accordance with the present 

 state of our knowledge, may feel himself impelled to visit this 

 region, which is so near and so accessible. Even if the trav- 

 eler should devote himself exclusively to geognostic investi- 

 gations, there still remains much to be done here — especially 

 the oryctognostic determination of the trachytic, doleritic, 

 and melaphyric rocks ; the separation of the primitive mass 

 upheaved, and of the portion of the elevated mass which has 

 been covered over by subsequent eruptions ; the seeking out 

 and recognition of true, slender, uninterrupted lava streams, 

 which are only too frequently confounded with accumulations 

 of erupted scoriae. Conical mountains, which have never 

 been opened, rising in a dome or bell-like form, such as Chim- 

 borazo, are, therefore, to be clearly separated from volcanoes 

 which have been or still are, active, throwing out scorias and 

 lava streams, like Vesuvius and ^tna, or scoriae and ashes 

 alone, like Pichincha or Cotopaxi. I know nothing that 

 promises to impart a more brilliant impetus to our knowl- 

 edge of volcanic activity, which is still very deficient in multi- 

 plicity of observations in large and connected continental dis- 

 tricts. As the material results of such a labor, collections 

 of rocks^would be brought home from many isolated true vol- 

 canoes and unopened trachytic cones, together with the non- 

 volcanic masses which have been broken through by both ; 

 the subsequent chemical analyses, and the chemico-ggological 

 inferences deduced from the analyses, would open a field 

 equally wide and fertile. Central America and Java have 

 the unmistakable superiority over Mexico, Quito, and Chili, 

 that in a greater space they exhibit the most variously-formed 

 and most closely-approximated stages of volcanic activity. 



At the point where the characteristic series of the volca- 

 noes of Central America terminates on the borders of Chiapa 

 with the volcano of Soconusco (lat. 16° 2^), there commences 

 a perfectly different system of volcanoes — the Mexican. The 

 isthmus of Huasacualco and Tehuantepec, so important for 

 the trade with the coast of the Pacific, like the state of Oaxa- 

 ca, situated to the northwest, is entirely without volcanoes, 

 and perhaps even destitute of unopened trachytic cones. It 

 is only at a distance of 160 geographical miles from the vol- 

 cano of Soconusco that the small volcano of Tuxtla rises, near 

 the coast of Alvarado (lat. 18° 28^. Situated on the eastern 

 slope of the Sierra de San Martin, it had a great eruption of 

 flames and ashes on the 2d of March, 1793. An exact astro- 

 nomical determination of the position of the colossal snowy 



