HALL’S EXCURSIONS, 343 
warm air, which ascends from the ravine of the volcano, has 
doubtless some influence in raising the general temperature. 
When a considerable portion of a mountain rises high enough 
to be covered with perpetual snow, the congealed mass has 
probably a contrary effect on the surrounding atmosphere, 
so that the line may descend somewhat lower than when a 
mere solitary peak is so elevated. We found the lowest 
limit of snow, on Cotopaxi, at 15,646: on Antisana, at 15,838: 
on Chimborazo, at 16,000: and on Cayambe, where a broad 
field of snow is spread over a gradual descent, as low as 
14,217 feet. Generally, on all the Cordillera, the snow 
descends much lower in summer, that is in the dry season, 
from June to October, than in the rainy season, or winter. 
We visited Cotopaxi and Chimborazo in November and 
December, and Cayambe in October: the two former, 
when the winter had set in, and the latter at the end of the 
dry season. 
Having finished our survey, we returned the same day to 
Quito, performing the descent in about seven hours. 
Five eruptions of Pichincha are recorded in the Annals 
of Quito; viz., in 1533, 1539, 1560, 1566, when consider- 
able masses of stone were carried down the ravine, adjoining 
Inca Pilca, into the plain, a little to the north of Quito, 
which is still called in the Quichua language, Rumi; , or 
“the plain of stones.” ‘This eruption is not easily reconciled 
with the present figure of the volcano, unless we suppose it, 
as is frequently the case, to bave broken out, not from the 
crater, but laterally from the neighbourhood of Gwagao 
Pichincha. In October, 1660, it rained ashes for several 
days in such abundance as to cover the country to a con- 
siderable distance, in memory of which event, the festival of 
the * Virgin of Mercies" is still celebrated on the 27th of 
October, to whose miraculous image was ascribed the pre- 
servation of the city. ; 
Excursion to the Obsidian Rocks of Quisca. 
M. Boussingault having been informed of the existence of 
