National Geographic : 1900 Sep
THE COLORADO DESERT As the way leads southward, Signal Mountain, the northern peak of the Cocopah range, disappears from view, but in front appears, ris ing from the plain, an isolated and striking landmark, the noble crater of the Sierra Prieta, called by Americans "Black Butte." Such active eruptions of lava as built up this perfect crater are indeed past, but secondary volcanic activity is still present in the hot mineral springs that surround the Colorado Desert and in the beautiful erup tive mud springs or salses, known as mud-volcanoes. These are found in two places on the Colorado Desert-in the Cocopah Valley a few miles from Sierra Prieta, and far north, just south of Salton Sea.* The whole region of the Sierra Prieta is full of evidence of recent action. North of the mountain and half a mile from its base are three hillocks, the largest 100 yards long and 50 feet high, which were evidently formed by the eruption of soft mud accompanied by gas. The rock of these mounds is imperfectly hardened mud, full of vesic ular cavities such as would be formed by the presence of gas in the mud eruption. The Sierra itself is several hundred feet high, and, with the deposits of broken lava that surround its base, has a circum ference of seven or eight miles. The rock is scoriaceous lava, with oc casional basaltic blocks that exhibit an imperfect culumnar structure. The rock of the sides is much broken by weathering, but the edge of the crater is beautifully defined. The floor of the depression is smooth and level and covered with fine clay, evidently blown in by winds. Water at times has stood at considerable depths within the crater. It is 250 paces across the perfectly circular bottom. At the center of the floor a small basin has been scooped out by human hands to collect the last drops of rain water. The mud-volcanoes lie on a flat, mud plain south of the Sierra Prieta, and during the overflow are surrounded by the waters of the New and Hardy Rivers and the "salt slough; " also a few of the springs are buried beneath the risen waters of Volcano Lake. At the time of my visit one of these springs was erupting beneath the water, throw ing up mud several feet above the surface with a cannonading that could be heard at a distance of three or four miles. Along the shores * For an interesting description of their discovery by Major Heintzelman, in 1852, see Pacific Railroad Reports, vol. v; "Geological Report," by Wm. P. Blake, p. 115. See also "An ac count of some volcanic springs in the Desert of the Colorado in Southern California," by Dr John L. Le Conte, American Journal of Science and Art, 2nd series, vol. xix, May, 1855, ana " Notes of a visit to the mud-volcanoes in the Colorado Desert in the month of July, 1857," by Dr John Veatch, in the same journal, vol. xxvi, p. 286, 1858, and also published in the Proceed ings of the California Academy of Science, 1857, p. 104.