Page:Popular Science Monthly Volume 71.djvu/192

186 stiff lava, being forced from the vent after the last eruption, hardened into a high mound. In the case of Pelée the shape of the mound was modified by a splitting off of the lava along vertical planes, which produced the unique "spike" of that volcano.

Age.—A comparison of this volcano with others in Mexico has led Ordoñez to state that it probably made its appearance during Pliocene times.

Cinder cones a few hundred feet in height are common objects in the central volcanic plateau of Mexico. Many of these may be seen in the basin in which the City of Mexico is situated, where the lower flanks of the higher volcanoes meet the plain. Near Toluca excellent examples occur. Because of the smallness of these cones as compared with the volcanoes near whose base they rise they are likely to be overlooked on

account of the overshadowing effect of the former. This is not true of the group of cinder cones, situated near the city Valle de Santiago, which are scattered about the valley some distance from the higher volcanoes, and which are, consequently, very conspicuous, their symmetrical truncated cones being the most marked features of the landscape.

This group of eleven craters occupies an area roughly circular in outline, one diameter of which is about six miles. Because of the fact that the valley of Santiago is a dry plain, the presence of lakes of pure water in four of the craters is unexpected. The clear blue