Page:The Scientific Monthly vol. 3.djvu/425

 EXPLOSION CRATERS

��4t9

��The craters of the Puebis' region are in two principal groups. One group of four is in the vicinity of the Hacienda and Sierra of Techa- chalco, about 26 miles north-northweat of Orizaba Peak and not far southwest of Limon Station on the Interoceanic Bailroad. The other group is near Alzozuca, a small settlement eighteen miles northwest of Orizaba Peak. The configuration and Btructure of two of the prin- cipal members of the first group are shown in Figs. 1 and 3, copied

���from Ordonez's memoir. The largest crater, known as Alchichica, is circular, contains a large shallow lake, and has a diameter of slightly more than a mile. The water level is about 50 feet below the sur- rounding plain. On the east and south sides the encircling ridge is low, but on the west it rises 335 feet. The inner slopes are steep, the outer ones more gradual. Both walls consist of ash, lapilll and pumice, evidently ejected from the crater, but on the west side these are seen to lie on a mound of basalt of some old flow. Fragments of basalt and of slates, limestones and other materials are included in the mass of ■ ' ' Lob Xalapascos del Estado de Puebla, ' ' Inst. Qeol. de Mexico, Parergones Vol. 1, No. B, 1805.

�� �