Nearly all of the volcanoes of the United States lie west of the meridian of Denver. These volcanoes belong to two distinct types, either the Vesuvian type with built up cones, or the plateau or fissure type already referred to. The following brief description of the volcanoes of the United States has been collated, for the greater part, from Wallace's excellent book on the volcanoes of North America. Crossing the United States on the Southern Pacific Railroad one's attention is caught, in Arizona, by a magnificent group of mountains known as the San Francisco Mountains. The highest peak of these mountains reaches 12,562 feet above the level of the sea, and 5,700 feet above the surface of the plateau on which the mountains stand. [Illustration: FIG. 18. THE UNITED STATES] According to G. K. Gilbert, the San Francisco Mountain group is formed of a variety of lava known as trachyte, that is of comparatively recent ejection, possibly of a geological age called the Tertiary. The lava forming the mountains escaped through a number of crater cones, some of which can still be seen in the neighborhood. Some of these craters are now in almost as perfect a condition as the day they were formed. Indeed, to one looking at them from a neighboring elevation, they appear so fresh, and so little affected by the climate, that one might almost believe that the lava had just flowed out of the craters, and has not yet hardened. Nevertheless, geologists are sure they have been formed long before man appeared on the earth. In one of these craters a lake of fresh water has collected. Another extinct volcano of the United States is Mt. Taylor in New Mexico, nearly east of the San Francisco Mountains. This mountain rises from the surface of a high table-land, or, as it is called in this part of the world, a _mesa_. The surface of the plateau is covered with a thick lava stream from which Mt. Taylor rises to a height of 11,390 feet above the level of the ocean. This mesa, or table-land, is forty-seven miles in length from northwest to southeast, and about twenty-three miles in breadth. Its general elevation is about 8,200 feet. The plateau rises about 2,000 feet above the surface of the level land that surrounds it. All these 2,000 feet have been removed by erosion. The table-land from which Mt. Taylor rises has not been eroded by the action of the rain, rivers, and other weathering agencies like the surface of the country surrounding it, because of a covering of lava that has been spread over its surface to a depth of about 300 feet. Mt. Taylor is formed almost entirely of lava that has escaped through a single opening and has built up a high cone around it. The volcano is now quite extinct, so that the original form of the mountain has been greatly changed by erosion.