POPOCATEPETL AND IXTACCIHUATL—FARRINGTON., gI 
lar depression between the Pico del Fraile and the eastern slope of the 
cone. As is indicated on the map (PI. XVIII) the stream has a course 
nearly north for some distance, then turns to the northeast and east, 
and empties its waters into the valley of Puebla. The barranca has 
a depth, where the trail crosses it, of about 60 feet. The strata exposed 
in section here have been fully described by Aguilera and Ordofiez* 
as follows: Below the black sand, which covers everything in this 
region, appears, first, a red breccia, made up of fragments of andesite: 
joined together by an argillaceous cement; second, a grayish breccia 
of pumiceous fragments less coherent; third, one of coarse fragments 
of andesite, and fourth, one of fine, pumiceous materials. These 
breccias show consolidation by percolating waters, but no sorting of 
materials or stratification such as would indicate deposition by water. 
They probably, therefore, lie very much in the order in which they 
were thrown out by’ the volcano. 
Crossing the barranca of Tlamacas the trail passed on in a gen- 
eral southeast direction over the layers of black sand or cinders which 
everywhere cover the cone. These cinders are simply comminuted 
fragments of the glassy, hypersthene andesite which makes up the 
mass of the cone. The particles are somewhat rounded by attrition, 
but show many edges so sharp that one must conclude that the sand 
is either of recent formation or else has not been much shifted by the 
winds. The degree of division varies considerably from that of the 
finest ashes to fragments as large as one’s fist. Some trials which I 
made indicate that the average coarseness of the sand is such that 
the largest portion will pass between a 35-mesh and 30-mesh sieve. 
There are areas where a much greater degree of coarseness prevails, 
and many bowlders scattered about are a cubic foot or more in dimen- 
sion. The rough, angular surfaces and scattered situation of these 
larger masses indicated that they were thrown out with the sand dur- 
ing explosive eruptions. Beyond the limit of trees, vegetation con- 
tinues over the slopes as a long, wiry grass to a point about 500 feet 
higher, although the sands blown about by the winds frequently 
cover the grass and choke it out. The determinations of the altitude 
of the limit of vegetation and snow line for the different portions of 
the mountain may be found in the following table: 


*Of, cit. pp. 12 and 37. 
