YOLCANO POPOCATEPETL AND ITS VICINITY. 



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feet). The snow extends much lower down during the summer (from June to Sep- 

 tember) than during the winter months (from October until May). This is caused 

 by the supply of moisture from the clouds, in which the mountain tops are wrapt 

 almost constantly during the rainy (or summer) season. Snow falls the greater 

 part of this time, and though the sun is very powerful, it rarely breaks through the 

 clouds. The snow, melted by the heat, is replaced almost immediately by a fresh 

 fall, whilst in winter, when the sky is generally clear, the accumulated snow is 

 melted to a great extent by the direct rays of the sun. 



Eeforo reaching the limit of perpetual snow, there appears a layer of transparent 

 ice beneath the upper strata of black sand. I observed the first traces of this ice at 

 a height of 14,000 feet. I could not ascertain its average thickness, although this 

 is evidently considerable. The sand lies from two to four feet deep above it. In 

 an attempt to ascend the peak of Orizaba, in September, 1856, I also noticed, near 

 the limit of perpetual snow, a layer of similar ice beneath the first stratum of sand. 

 This seems to prove that, at the time when the last eruption occurred, the snow 

 extended as low or lower down than the ice does at present. The ejected sand, 

 still warm when it reached the ground, woidd melt the surface of the snow, and 

 the water, thus produced, penetrating into the lower layers of snow, would freeze 

 again, changing the strata with which it is mixed into a kind of glacier ice. 

 This ice, protected from the solar heat by the covering of sand, is not liable to 

 thaw, and that part of the rain or melted snow, which filters every year through 

 the layer of sand, freezes as soon as it comes in contact with the stratum of ice, 

 thus increasing its bulk; and, as at this height the mean temperature of the year 

 is below the freezing point, it is not probable that it woidd ever melt again. 



A deep gully or ravine, the head of which is about 16,000 feet above the level of 

 the sea, issues from between the highest or western peak and the " Pico del fraile.'' 

 It runs for some distance in a N. N. E. direction, turning afterwards more to the 

 east. This gully exposes the strata as they were deposited just on the foot of the 

 volcanic cone by many successive eruptions. Tlie lower beds are of a light 

 brownish-red or of a yellowish-white color, and consist of pumice and scorite, while 

 the last six layers are alternately pumice and black sand. From this it would 

 appear that the vast mantle of black sand, which covers the cone as well as the 

 slopes of the mountain for many miles, is the product of the last efforts of the 

 expiring volcano.' 



After leaving the " Rancho Tlamacas" it is still possible to ride for a considerable 

 distance on horseback, and travellers generally dismount and leave their horses only 

 a few hundred yards below the limit of perpetual snow. The animals can, of 

 course, only make slow progress in ascending the steep slopes of the mountain, as 

 they are obliged to walk in deep sand, affording no secure footing, and often slide 

 back as much as they advance; but the traveller, in riding, saves his strength mean- 

 while for the toil of ascending the last 3500 feet. 



It is always best to get upon the snow as soon as possible after leaving the horses, 

 because this affords a inuch better footing tlian the loose sand. The eyes should 



' This passage was copied from one of Prof Monross' notes. 



