52 



NOTES ON THE 



which have nearly the same elevation above the level of the sea as the plain of 

 Amecameca. 



The most convenient starting point for ascending the mountains is the town of 

 Amecameca, situated in the plain which takes its name from it, at the eastern 

 base of a hill, called &quot; Sacremonte.&quot; A chapel stands upon the summit and another 

 on the eastern slope of this hill, which is about 400 feet above the plain. At the 

 town seven or eight Indians should be engaged as guides and to man the capstan, 

 by which, with a rope attached, the traveller is lowered down into the crater. The 

 first spurs of the ridge are reached a mile east of the town. They rise very gently 

 for a considerable distance, but gradually become steeper and steeper. Whore 

 they are intersected by small ravines or gullies, washed out by the torrents, there 

 appear just beneath the surface alternate layers of yellow pumice and black 

 volcanic sand, in which boulders of feldspathic and porphyritic lava are entirely or 

 partially imbedded. The surface in the upper regions is covered with a long 

 wiry grass, and studded with pine-trees. Boulders of various sizes and shapes are 

 scattered over it in all directions. 



No lava streams appear to have run down on the west side of the mountain, and 

 the absence of such streams is explained by the fact, that the western wall of the 

 crater is nearly 600 feet higher than the eastern rim. A similar elevation of the 

 western or southwestern wall above the opposite, as far as I have noticed, exists in 

 all recent Mexican craters. It is particularly well defined on the peak of Orizaba, 

 where even from Vera Cruz a horizontal black line is seen just below the highest 

 point, strongly contrasting with the snow around it : this line is the western wall 

 of the crater. 



The volcano of the Colima, and the volcano of Toluca also, have their highest 

 points at the Avcst sides of their craters. The same peculiarity marks the outline 

 of a volcano of very extraordinary form, situated in the valley of Mexico, about 

 twelve miles S. E. by E. from the city, near the village of Ayotla, and called the 

 volcano of San Isidro : it has two distinct craters ; the eastern is much lower than 

 the western, and divided from it by a wall of very compact lava ; the highest 

 point about 1500 feet above the level of the lakes is W. S. W. from the western 

 crater. The bottoms of the craters of this volcano are at present cornfields. 



FIG. 1. 



VOLCANO OF SAN ISIDRO, SEEN FROM N. N. W. 



I might add many more instances, in which I have observed the western walls 

 of the craters of Mexican volcanoes to be considerably higher than their eastern 

 edges, but it will be sufficient to state that, in no case, have I seen a volcanic cone 

 whose crater is of recent formation, the east side of which was higher than the 



