[Illustration: FIG. 20. VOLCANIC NECKS, EDGE OF MESA AT MT. TAYLOR _From U. S. Geological Survey_]
A similar lake, known as Lake Lehontan, existed at the same time, covering large areas in the western parts of Nevada.
Coming now to Ice Springs Craters in Utah, we find here three small craters formed of scoriæ and lapilli (volcanic ashes consisting of small angular stony fragments). Near them lies a fragment of a much larger crater known as the Crescent. In some respects this crater was not unlike the crater of Somma that surrounded Mt. Vesuvius. It was not, however, as large, having a diameter of only 2,200 feet. From these craters streams of basalt flowed until they covered considerable areas.
A still more recent crater known as Tabernacle Crater is situated four miles south of the Ice Springs Crater. Tabernacle Crater takes its name from the building known in Salt Lake City as the Tabernacle. According to Gilbert, this crater was formed at a time when Lake Bonneville stood at a comparatively low level, or when the water was only from fifty to seventy-five feet above the bottom of the valley on which the crater now stands. At that time an explosive volcanic eruption occurred on the bottom of the lake, and the rim of the crater, built up by this explosion, was gradually pushed above the surface of the lake, so as to shut out its waters.
Extinct volcanic craters, not unlike those of Utah, occur also near Ragtown, in Nevada, in a district known as the Carson Valley Desert, in one of the broadest areas of what was once Lake Lahontan. Ragtown is twenty-two miles southwest of Wadsworth on the Central Pacific Railroad. At the present time there are two circular depressions or volcanic craters filled with pools of strongly alkaline water known as the Ragtown Pond, or Soda Lake. The large lake covers an area of 268-1/2 acres. Its greatest diameter is over 4,000 feet. Without going into a detailed description it will suffice to say that the larger crater probably was destroyed by an explosive volcanic eruption.
Another intensely alkaline lake that fills an extinct volcanic crater is the Mono Lake, situated in Mono Valley in California at the eastern base of the Sierra Nevadas. It has an area of about 200 square miles. The centre of the lake has two small islands named Pacha and Negit. Immediately south of Mono Lake are a number of craters that occupy portions of what was once apparently a fissure extending in a general north and south direction. The highest of these craters are in the neighborhood of 2,500 feet.