Plate 10.—(a) Lassen Peak, Northern California, in Eruption August 22, 1914. The great cloud of steam and volcanic ash rose several miles. This is the only active volcano in the United States proper, and it is now included in Lassen Volcano National Park. (By permission of R. E. Stinson, Red Bluff, Cal.)
Plate 10.—(b) Devil’s Tower, Wyoming. This great mass of rock was forced in molten condition through strata which, because of their weakness, have been eroded away all around the hard igneous rock. This is probably the core or neck of a former volcano. (Photo by Darton, U. S. Geological Survey.)
Fig. 31.—Diagram showing the origin and character of a spiral nebula according to the planetesimal hypothesis of the origin of the solar system. (Modified after Moulton.)
Anything like a full understanding of the planetesimal hypothesis would be difficult to obtain, and, in the brief space at our disposal, we shall attempt to make clear only a few of the salient points. According to this hypothesis the solar system was, during a previous stage of its evolution, a great, flat, spiral nebula, made up of finely divided solid or possibly liquid particles called planetesimals, among which were scattered some larger “knots” or masses. Each tiny particle and larger mass or knot is considered to have traveled in its own particular orbit or path about a central very large mass—the future sun. It is even suggested that the spiral nebula originated by disruption of one star by a swift-moving passing star. Each disrupted particle and large mass at first started straight for the large passing star, but because of change of position of the latter the particles and larger masses were gradually pulled around so that their paths curved into spirals. Because of crossing of paths, the larger masses or knots gradually increased in size by accretion of the small particles or planetesimals. Meteors (so-called “shooting stars”) which now strike the earth are thought to be disrupted materials still being gathered in, though very slowly at present. After the passing star got well out of range, the spiral paths of the disrupted masses gradually changed to nearly circular, due to a wrapping-up process around the central body (sun) which then controlled the movements of the both larger masses (future planets) and small masses (planetesimals).