ON ITS EXTERIOK. VOLCANOES. 315 



Chimborazo (first published in 1837 in Schumacher's 

 Astronomisches Jahrbuch), I had expressed this con- 

 jecture, when speaking of the remarkable pieces of 

 augitic porphyry which, on the 23rd June 1802, I col- 

 lected in loose fragments of twelve or fifteen inches 

 diameter at a height of 19,000 feet, on the narrow 

 rocky ridge leading to the summit. They were porous, 

 and of a reddish colour, minutely cellular, and the cells 

 shining. Some had a blackish tinge, and in some 

 cases were of very light weight, like pumice, and looked 

 as if freshly altered by fire. They have not, however, 

 flowed lava-like in currents, but have probably been 

 expelled from fissures on the declivity of the pre- 

 viously upheaved mountain dome. This explanation 

 of their origin would find great support in the view 

 entertained by Boussingault, who considered the vol- 

 canic cone itself to be " a heap of angular fragments 

 of trachyte, upheaved in a solid state, and piled upon 

 each other without any definite arrangement. As a 

 heap of broken fragments of rock occupy more space 

 than would the same quantity of material before being 

 fractured, there remain large intervening hollow spaces 

 in which, by mutual pressure and shock (apart from the 

 effect of the force of volcanic vapours), movement 

 arises." I am far from doubting the occurrence of 

 such fragments and of such hollows, which in the 

 Nevados became filled with water, although the fine, 

 regular, and generally quite vertical, columns of tra- 

 chyte of the Pico de los Ladrillos and Tablahuma on 

 the Pichincha, and, most of all, those above the little 

 lake of Yana Cocha, on Chimborazo, appear to me to 

 have been formed on the spot. My long and dearly 



