REVIEWS sy) 
irregular and tortuous fold of the Sierra. Several great barrancas or 
ravines made by the flow of waters from above cut the lower portion 
of the cone and modify somewhat its relief. They take their origin 
in semicircular depressions of the cone which have a more or less 
crateriform aspect. The most notable of these is that at the head of 
the Barranca del Fraile. It cuts a great amphitheater-like section 
out of the northwest flank of the mountain and is surmounted by a 
peak known as the Pico del Fraile. 
This great depression has usually been regarded as an old crater, 
but in the view of the authors it is simply a cavity cut in the moun- 
tain by atmospheric and stream erosion. ‘Their reason for this view is 
that the slope of the strata of lava is everywhere uniform with that of 
the major cone. Had the lava exuded from this point they argue, 
the slope of the layers would have been away from the crater on all 
sides. 
The form of the crater at the summit of the mountain is that of an 
elliptical cylinder resting upon an inverted cone. 
Its greatest depth is 1640 feet (505 meters), greatest diameter 1988 
feet (612 meters) and least diameter 1299 feet (400 meters). These 
determinations, especially that of the depth, differ considerably from 
those made by previous observers. The form of the crater, the authors 
state, was probably originally that of an inverted cone. ‘Their expla- 
nation of its present form is that by the accumulation of material dis- 
lodged from its walls, the interior has been filled up and the outer 
border extended till the interior walls are now nearly vertical. 
In regard to its structure, Popocatapetl is classed with the volcanoes 
known as stratified, on account of the resemblance to sedimentary for- 
mations presented by the succession of currents of lava of which it is 
made up. 
In the character of the lava of these successive outflows, a marked 
gradation can be traced, corresponding to a gradual decrease in the 
heat and energy of the volcano. ‘The lavas of the lower currents are 
granular, devitrified and of trachytic structure; those of the upper 
currents, vitreous, of resinous luster and amorphous structure with a 
few phenocrysts. The difference in structure is ascribed to the fact 
that the upper currents being farther from the source of heat were 
more nearly cooled when poured out and became solid before time 
sufficed for crystallization. 
A difference in mineralogical composition between the upper and 
