ON ITS EXTERIOR. VOLCANOES. 267 



any former period emitted scoriae and lava, like Vesuvius 

 and Etna, or scoriae and ashes only, like Pichinca and 

 Cotopaxi. I do not know anything which holds out a 

 better promise of a brilliant advance in our knowledge 

 of volcanic activity, in which sufficiently varied observa- 

 tions extending over large and connected continental 

 spaces are still greatly wanting. And further, supposing 

 collections of rock-specimens to be obtained and brought 

 home from several isolated true volcanoes, and from un- 

 opened trachytic cones, and also from the non-volcanic 

 masses which have been broken through by them, the 

 chemical analysis to which the specimens would be 

 submitted, and the chemico-geological inferences which 

 would be drawn from the analysis, would open a no less 

 wide and fruitful field. Central America and Java have 

 both the undeniable advantage over Mexico, Quito, and 

 Chili, of offering the most varied and the most crowded 

 examples of " scaffoldings " in which volcanic activity 

 is displayed over large areas. 



At the point, in lat. 16 2' K., where at the boundary of 

 Chiapa the very characteristic series of the Central Ame- 

 rican volcanoes terminates in the volcano of Soconusco, 

 a wholly different system of volcanoes, the Mexican one, 

 begins. The isthmus of Huasacualco and Tehuantepec, 

 so important for future commerce with the coasts of 

 the Pacific, as well as the more north-westerly State of 

 Oaxaca, is entirely without volcanoes, and perhaps also 

 without unopened trachytic cones. It is not until after 

 a distance of 160 miles from Soconusco, that the small 

 volcano of Turtla, in lat. 18 28', rises on the coast of 

 Alvarado. Situated on the eastern declivity of the 

 Sierra de San Martin, it had a great eruption of flames 



