What is the bearing of this nebular hypothesis upon the early geological history of the earth? According to the hypothesis the earth must once have been much more highly heated and larger than now. It condensed to a liquid and then it cooled enough to permit the formation of a solid crust over a liquid interior. It then had a hot dense atmosphere containing all the water of the earth in the form of vapor, and this atmosphere steadily became thinner due to absorption by the earth. When the pressure and temperature conditions became favorable, much of the water vapor condensed to form the ocean and the atmosphere gradually changed to its present condition. According to this view the oldest rocks of the earth must have been igneous because they resulted from the solidification of the outer part of the molten globe. Within recent years certain serious objections to the nebular hypothesis have been raised, and Chamberlin and Moulton have formulated the planetesimal or spiral hypothesis as an attempt at a more rational explanation of the origin of the solar system. Some of the objections to the older doctrine are that among the many thousands of known nebulæ in the universe very few only are of the Laplacian or ring type, while spiral forms are abundant. Spectroscopic study shows that the nebulæ are not gaseous, but made up of either liquid or solid particles, and that the leaving off of rings would necessitate the assumption of an intermittent process which could scarcely have operated under the conditions of the hypothesis. [Illustration: Plate 9.--(_a_) Molten Lava Flowing Over a Cliff Into Water in the Hawaiian Islands. (_After Diller, U. S. Geological Survey._)] [Illustration: Plate 9.--(_b_) Dikes of Granite (Light Gray) Cutting an Old Dark Rock. While the granite on the right was being forced in molten condition upward into the earth's crust, tongues of it (dikes) were sent off into the adjacent rock. (_Photo by Howe, U. S. Geological Survey._)] [Illustration: 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._)] [Illustration: 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._)] [Illustration: 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.)]