Page:The Scientific Monthly vol. 3.djvu/427

 EXPLOSION CRATERS

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��The craters about Alxoxuca are on the plain at the foot of a vol- canic ridge, but the materials in the walls and slopes are entirely of the tuffs which make up the plain. The craters are close to cones from which lava flowed recently, at one place overiapping the ejecta from the explosion craters. The principal crater, known as Alxoxuca, is somewhat more than a half mile in diameter, and the lake which it contains is 200 feet deep. The cross section, Fig. 3, shows the prin- cipal features on a west to east line. Two lava sheets are intercalated in the tuff on the east side, which rises considerably higher than the west side. The encircling rim consists, as in other craters, of fine ejecta containing many basalt fragments. The deep large crater of Tecuitlapa contains, as shown in Fig. 4, a conical mass of basalt and

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��Fig. 4. Sbction Acboss ths Explosion Cbatbr of Tbcuitlapa in Pubbla, Mbzico,

AFIBB OBDONBZ.

breccia with three small craters, a feature very similar to Zuni Lake in New Mexico. Steep walls rise from the central lake to the crest of the encircling ridge of ejecta which is 300 feet high and slopes gently to the adjoining plain. The interior cone and its craters are clearly of more recent age than the large crater and undoubtedly represent a final stage of the volcanic activity.

Two other craters, five miles east of Alxoxuca, are known as Xala- pazco Grande and Xalapazco Chico. They contain no water. As shown in Fig. 5, they are less than a half mile in diameter, but the

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����Fia. 6. A. Section Acboss thb Cbateb, Axalafazco Gbandb. B. Section Acboss

THE CBATEB, AXAIiAPAZCO CHICO, PUEBLA, MEXICO, AFTEB OBDONEZ.

��encircling rim does not rise high above the plain. The larger crater is 220 feet deep, and the smaller one nearly 400 feet. As in the case of the others, they are blown out through andesitic tuffs constituting the general valley fioor and the rims consist of a large volume of fine ejecta containing much basaltic material. Ordonez suggests that at the time of eruption a large volume of water was ejected with the

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