water of the lakes with their settings of green cultivated fields which cover the inner slopes of the craters are most beautiful objects.
The existence of these lakes is due to the fact that their bottoms are below the levels of underground water. All these crater lakes are at practically the same level, a condition which is due to the fact that the volcanic material in which they rest and of which the plain is composed is extremely porous, which permits the free circulation of the water. The craters of the majority of the cones were partially filled with lava which poured out quietly after the explosions which formed them had ceased. In some cases they were filled until their bottoms were above the level of underground water and are consequently dry; in others there was either no subsequent outpouring of lava or the quantity was very limited, in which case the cavity remained below the level of underground water and a lake resulted. The diameter of the craters vary in size from that of Solis (1,500 feet)—which was apparently produced by the sinking of the crust—to the largest, which is more than a mile in diameter. The craters are not all perfect; some are entire, while others are broken by one or two subsequent craters of explosion. In one of these breeched craters three small cones rise from the bottom, the material of which is apparently being used in the city for constructional purposes.
The plain upon which the craters rest is underlaid by one or more strata of basaltic lava which evidently flowed from the neighboring mountains and which may be seen near the water level of the lakes and in ravines which have been deeply cut by streams. Since neither this stratum nor the strata of basaltic lava are disturbed by being domed up or bent to any extent, it seems safe to conclude that the explosions forming the craters must have been near the surface and very local, otherwise the strata overlying the plain at that place would have been more or less bent.
The cones are made up in some cases of volcanic ash of various degrees of fineness, in others of volcanic breccia. The slopes are those which are normally made by such materials.
Because of the fact that craters of explosion in other parts of Mexico—Puebla, Mexico City, here in Valle de Santiago, and elsewhere in the republic—arise from a plain or a more or less enclosed basin which is full of water at a comparatively shallow depth, Ordoñez suggests that superficial water may have had a share in the production of the explosions.
Such are a few of the points of interest on the volcanic plateau of Mexico, a region which, interesting because of its scenery and climate, fascinating because of its romantic history, is to the geologist a volume which which studied will explain many points that are now a matter of speculation.
