[272] Of like import is the statement of Woodward—“If the crust of the earth were self-supporting, its crushing strength would have to be about thirty times that of the best cast steel, or five hundred to one thousand times that of granite.” Mathematical Theories of the Earth, Proc. Am. Assoc. for Adv. Sci., 1889, p. 49.
[273] It is assumed that the direction of the supporting thrust at the periphery of the dome is at every point parallel to the tangent to the domed surface. This is justified by symmetry in the case of a shell conforming to the sphericity of the earth, and in the other cases it would seem to be as favorable an assumption in the direction of high supporting capacity as can reasonably be made.
[274] Prepared at the authors’ request by W. H. Emmons.
[275] The terms are here used in their narrow technical sense. Extrusion is also used in a broad generic sense to indicate the whole process of outward movement.
[276] Gilbert. 14th Ann. Rept. U. S. Geol. Surv., Pt. I, p. 187.
[277] Gilbert, after a careful study of the moon’s topography, has suggested that the lunar pits may be indentations produced by infalling meteorites or planetoids, and has shown by experiment that pits of a similar type, with similar central cones, can be produced by impact. The Moon’s Face: A Study of the Origin of its Features. Presidential address, Phil. Soc. of Washington, 1892, Bull. Vol. XII, pp. 241–292.
[278] Structure and Distribution of Coral Islands.
[279] Corals and Coral Islands.
[280] Proc. Roy. Soc. Edin., Vol. X, pp. 505–18, and Vol. XVII, pp. 79–109; Nature, Vol. XXXII, p. 613; Narrative Chal. Exp., Vol. I, pp. 781–2.
[281] Bull. Mus. Comp. Zool., Vol. XVII, 1889.