How important is the cover of vegetation in retaining the rock mantle and the upper soil layer in their respective positions, as required for agricultural purposes, may be best illustrated by the disastrous consequences of allowing it to be destroyed. Wherever, by the destruction of forests, by the excessive grazing of animals, or by other causes, the mat of turf has been destroyed, the surface is opened in gullies by the first hard rain, and the fertile layer of soil is carried from the slopes and distributed with the coarser mantle upon the bottom lands. Thus the face of the country is completely transformed from fertile hills into the most desolate of deserts where no spear of grass is to be seen and no animal food to be obtained (plate 5 A). The soil once washed away is not again renewed, for the continuation of the gullying process now effectively prevents its accumulation. ┌────────────────────────────────────────────────────────────────┐ │ PLATE 5. │ │ │ │ [Illustration: _A._ Once wooded region in China now reduced to │ │ desert │ │ through deforestation (after Willis).] │ │ │ │ [Illustration: _B._ “Bad Lands” in the Colorado Desert (after │ │ Mendenhall).] │ └────────────────────────────────────────────────────────────────┘ READING REFERENCES TO CHAPTER XI Decomposition and disintegration:— GEORGE P. MERRILL. The Principles of Rock Weathering, Jour. Geol., vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and Soils. Macmillan, New York, 1897, Pt. iii, pp. 172-411. ALEXIS A. JULIEN. On the Geological Action of the Humus Acids, Proc. Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410. Corrosion of rocks:— C. W. HAYES. Solution of Silica under Atmospheric Conditions, Bull. Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19. M. L. FULLER. Etching of Quartz in the Interior of Conglomerates, Jour. Geol., vol. 10, 1902, pp. 815-821. C. H. SMYTH, JR. Replacement of Quartz by Pyrites and Corrosion of Quartz Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285. Dome structure of granite masses:— G. K. GILBERT. Domes and Dome Structure of the High Sierra, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4. RALPH ARNOLD. Dome Structure in Conglomerate, _ibid._, vol. 18, 1907, pp. 615-616. Soil flow:— J. GUNNAR ANDERSSON. Solifluction, a Component of Subaërial Denudation, Jour. Geol., vol. 14, 1906, pp. 91-112. OTTO NORDENSKIÖLD. Die Polarwelt und ihre Nachbarländer, Leipzig, 1909, pp. 60-65. ERNEST HOWE. Landslides in the San Juan Mountains, Colorado, etc., Prof. Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20.