Page:The Scientific Monthly vol. 3.djvu/426

 420 THE SCIENTIFIC MONTHLY

ejecta constituting the encircling ridge. Theie is no evidence of dis- turbance of the beds.

Atexcaqui^ six miles southwest of Alchichica^ has an elliptical crater about three fifths of a mile wide from east to west and two fifths of a mile wide from north to south. Some of its features axe diown in Fig. 2. The surface of the lake occupying the crater is about 90 feet below the adjoining plain. The encircling ridge is from 200 to 250 feet above the lake^ but on the northwest side its height increases to about 600 feet on account of the presence of a mound of horizontal Cretaceous strata on which the ejecta is piled. On the other sides are low walls of basalt^ capped by nearly horizontal beds of yellow, andesitic tuff which constitutes the surface of the adjoining plains. The ma- terial ejected from the hole and constituting the encircling ridge is piled up on this tuff to a thickness of 40 to 60 feet. The inner slopes of the crater are steep at most places. The ejecta of the encircling ridge is in irregular sheets sloping outward at a low angle. The prin- cipal materials are those which underlie the plain: ash, cinders, tuff, and numerous fragments of the basalt. There is also a small propor- tion of limestone, granite, diorite and andesite, such as probably under- lie the locality at considerable depth. The limestone is of the same sort as that which constitutes the hill on the northwest side of the crater. The explosion has blown a round hole through the basalt, limestone and overlying tuff, but has not disturbed the remaining edges. The volume of ejected material is not estimated, but it must be great and apparently sufficient to fill the hole.

��Fig. 8. Section Across the Explosion Cbateb of Alxoxuca in Pubbia, Msxico,

AFTEB OBDONBZ.

La Preciosa, a smaller crater with lower surrounding ridge, also contains a lake said to be 275 feet deep. Its rim consists of a mass of ejected material somewhat irregularly distributed. The hole is in the tuff of the plain. Quecholac, which lies out on the plain some distance from the others, is about three fifths of a mile in diameter, with low encircling ridge 80 to 160 feet high. This ridge consists of ejecta, mainly cinders, lapilli and pumice, and a few rocks of the older forma- tions in irregular sheets dipping gently outward. The underlying strata are not exposed. The lake is 300 feet deep.

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