110 FirLp COLUMBIAN MusrEUM—GEOLOoGY, VOL. I. 
of the mountain bold escarpments to the west. The face of these 
escarpments was well-nigh vertical and strongly pillared, showing 
the columnar structure so characteristic of basaltic outflows. On the 
view given in Pl. XIV, a number of these escarpments may be traced 
on the flanks of the mountain. They apparently mark the course of 
outflows of eruptive rock of high viscosity, which, owing to this 
property, solidified before becoming-much distributed. The surface 
relief of the flows has, of course, since been modified to a considerable 
extent by erosion. 
The structure of many of these Paes shows that they were 
formed. by successive outflows of lava. Layers of scoriaceous 
rock alternate with breccias and rock of a more dense, compact 
character. Occasionally the scoriaceous layer could be seen to have 
been consolidated into a breccia by the subsequent superposed flow. 
This was finely exhibited in the ridge from which the view shown in 
Pl. XIV was taken, the general rock structure being quite similar to 
that observed on Popocatepetl. Another feature noticeable in this 
vicinity was that of several caves in the rocks, which may. have orig- 
inated as bubbles formed by the imprisoning of steam or vapors when 
the rock was still in a molten condition, but’ more probably were 
formed by the weathering out of a more easily decomposable stratum. 
They at present furnish shelter for cattle and for travelers making an 
ascent of the mountain. 
In one of these our party spent the night. The cave was reached 
about 5 Pp. M., the ascent from Ameca having consumed a little more 
than seven hours. Here the night was spent with as much comfort 
as possible, but after the going down of the sun the cold and wind 
were so severe that in spite of the protection of several blankets sleep 
was practically impossible. The next morning the ascent was con- 
tinued on horseback to the foot of the glacier, which was reached in 
about an hour. It was no part of my purpose to attempt to ascend 
the mountain, so no effort of this sort was made. All the time at 
command was devoted to observation of the features of the glacier, 
the results of which follow. The time permitted for these observa- 
tions was much less than could have been.desired, but sufficient 
to observe the main features of the glacier. 
Shortly after noon a driving, blinding snowstorm descended 
upon the mountain, preventing further observation, and compelling 
myself and guides to return to the camp as quickly as possible. As 
provisions were not at hand to enable us to spend another night on 
the mountain, return was made to eos which was reached without 
mishap about dusk. 
