POPOCATEPETL AND IXTACCIHUATL—FARRINGTON. 93 
through the side of the cone took place here. The topography of the 
surface is certainly such as to favor this latter view, and as will be 
noted later, considerable difference can be seen microscopically, 
between the rocks of the two regions. 
Another peculiarity of the rocks of the region has given rise to 
differences of opinion as toits origin. This is the flat, polished surfaces 
to be observed truncating the solid angles of projecting masses of the 
lava. According to Felix and Lenk,* since these surfaces dis- 
play no scratching or grooving, and appear on all sides of the rock 
masses, they can not be due to the action of snow or ice, but are the 
result of AZolian action, i.e., the movement of the sand-bearing winds. 
Aguilera and Ordofiez,+ however, finding no relation between the 
direction of the prevailing winds and the position of the polished sur- 
faces, and noting that the polishing is as complete in places sheltered 
from the winds as in those exposed to them, regard the smoothing as 
brought about by the action of ice and descending waters. My own 
opportunity for observing these surfaces was not sufficient to enable 
me to form an opinion as to their probable origin. 
Arriving at La Cruz, the horses and mounted guide were sent 
back to the ranch and arrangements were made to continue the ascent 
onfoot. This required fitting the feet with guvarachos or thick, broad 
’ sandals of heavy leather, around which several layers of burlap were 
wrapped. These insured warmth, but were worn primarily for pro- 
tection to the shoe soles. So destructive are the ice and ashes to 
ordinary leather that a pair even of the gwarachos is worn out in a 
single ascent. As this was the dry season the snow line was not 
encountered till we were several hundred feet above LaCruz. During 
the wet season, however, the snow and ice descend nearly to the 
Ranch of Tlamacas. Though the surface of the cone above the snow 
line is in general snow-covered, there are many long tongues of sand 
exposed on the lower parts of the slope. Upon these, so far as pos- 
sible, we made our ascent, for, made firm by the freezing of the water 
with which they are saturated, the sand slopes afford a better foothold 
than the rough snow and ice. Over this surface we toiled until only 
a white field of snow separated us from the summit. To advance 
over this, every step had to be cut in advance by the guide. The ver- 
tical rays of the tropical sun cause the melting of the snow to take 
place in a direction parallel to the rays, but at an acute angle with 
the slopes. Then while the sun’s rays cut the snow surface vertically, 
the rills formed from the melting snow waters cut it laterally. The 

*O%. cit. p. 24. 
tOZ. cit. p. 33. 
