Page:Popular Science Monthly Volume 49.djvu/501

Rh that produced the lake's annihilation, but in the main the aërial discharges of volcanoes. Even to-day the practiced eye will soon pick out from among the many mountain forms that surround this ranchland the conical contours of the volcano. A few such stand by themselves, neither of great height nor of imposing mass; others are disposed in linear series, much like the cones which so abundantly scatter themselves over the southwestern and Mexican plateaus. We ascended one of these, a conelet of perhaps one hundred and fifty feet elevation, whose partially wooded sides were yet the ancient slag and cinders, and from whose top projected the plug of lava which marked the position of the former vent. In a pit near by could be seen the hard



basalt-trachyte which forms the existing core of the mountain—the material which in early Miocene times, or perhaps still earlier, was active in the distribution of the loose rock fragments which everywhere lie scattered about. In the days of its activity the foot of the volcano bordered a still more ancient lake, or was even immersed in it, as the lacustrine deposits which largely encircle it plainly show. In these are found in scattered spots a number of fresh-water types of mollusks—Planorbis, Physa, Limnea, Valvata, Cyclas—their shells as beautifully preserved as the much more delicate parts of the insects which were shortly added to them.

The eruption came, and with it clouds of ash sailed upward, only to fall back into the lake waters, and with them form a sticky and lasting paste, ultimately to harden into a compact