An expedition, consisting of Mr. WA Cogshall and the writer, accompanied part way by Dr. T.].]. See and his brother, who went for a brief outing in the mountains, left Mexico for the as-· cent of the great volcano on April 12 of this year. Horses and mules were procured at Amecameca, and the fourteen-mile ride from there to the sulphur-smelting works, at a locality called Tlamacas, at the foot of the mountain, was made in five hours, from 2 to 7 P. i\I. This is an ascent from 8200 feet (2532 metres) to 12,900 feet (3937 metres), hut no one experienced any marked inconvenience from the altitude, although the increasing cold after nightfall became very disagreeable. Very little was eaten by us that night or the next morning, to avoid the indigestion which high elevation is apt to bring on: vVe left Tlamacas at 5 AM, on horseback, and at 6: 30 reached Las Cruces, at an altitude of about 14,000 feet, whence we proceeded on foot, as is customary. The path is on the northern slope of the cone, and at the time of our trip was but a short distance east of a large field of ice and snow impossible to traverse. On the entire ascent to the crater no snow was encountered, although in some places there were traces of it under the sand, lending to the latter its solidity, and greatly helping us. Mr. Cogshall reached the crater in 411 10m from the time be left Las Cruces. I was longer, taking fully 411 50111• Frequent r ('~ ts were necessary to regain breath and allow the pulse to quiet down. An attempt to climb more quickly brought on a very disagreeable but temporary feeling of exhaustion and inability to climb higher. One should be very careful of this, and, on the first sensation of overdoing, rest until entirely relieved, and then go more slowly.* Timing one's self with a watch, taking twenty or thirty steps, and then resting one minute, is the correct way to make a climb of such magnitude at high altitudes. The view of the crater is strikingly sudden. When I was approaching it without, however, seeing any signs, my guide, who