Thus, only to cite two examples from recent authors of justly high reputation. Lyell says of the probable subterranean sources, whether of upward or downward movement, when permanently uplifting a country, and in reference to the crumpling of strata on mountain flanks by lateral pressure, it would be rash to assume these able to resist a power of such stupendous energy, "_if its direction, instead of being vertical_, happened to be oblique or horizontal." This is somewhat vague--and I trust I do not mistake or misrepresent the illustrious author--yet it is the most explicit expression I can find in the "Principles of Geology" as to his notion of the primary direction of elevatory force (Edit. 10, Vol. I., p. 133). That Mr. Scrope's idea is that only of primary radial or vertical direction of such forces, is apparent on inspecting his Diagram No. 64 ("Volcanoes," p. 285), and in the use of the words, "an axial wedge of granite," which, on the next page, we find is "liquefied granite;" and if we read on to page 294, and refer also to pages 50 and 51, I believe there can be no doubt that _vertical_ or _direct up-thrust_ is the author's notion of the primary direction of all forces of elevation. The true nature of these forces was, however, clearly seen and most justly stated by Constant Prevost ("Compt. Rend.," Tome XXXI., 1850, and "Bulletin de la Société Géolog. de France," Tome II., 1840) as consisting, not in forces of some unknown origin acting primarily in the vertical, but in _tangential pressures acting horizontally, and resolved by mutual pressures at certain points into vertical resultants_. These Prevost rightly attributed to the contraction of the earth's solid crust. The same idea has been adopted by Elie de Beaumont as the true mechanism of the elevation of mountain ranges; and although De Beaumont's views as to the thinness he assigns to the solid and contracting crust, and his strange deduction as to the parallelism of contemporaneous mountain chains uplifted by its spasmodic action along certain lines, may be untenable, his notion generally as to the play of forces producing mountain elevation is much more nearly correct.