GEOLOGIC LANDSCAPES OF THE ABYSSINIAN AND INDIAN SURVEYS (1860S & 1870S) AS A CHALLENGE TO GRADUALISM

The Geological Survey of India was inaugurated in the mid-19^th century. It brought to bear the talents of an extraordinary group of geologists/naturalists. Their appointed tasks included evaluation of available coal and mineral wealth as well as general geological and natural resources. William T. Blanford, employed as a Survey geologist, engages our attention because of the works he authored, the images incorporated therein, and his changing notions as they related to geological processes across time. Among the terrains Blanford and his associates explored in India were the northwestern regions, the Punjab and Sind, where, as in desert regions universal, rock outcrops are generally more readily visible than in temperate and tropical areas, and the almost elemental impacts of erosion, especially by wind and water, are readily seen. In 1867, Blanford was assigned to General Napier’s Abyssinian military unit as geologist/naturalist. In due course, he drafted a geological map of the region, which contains a surprising amount of detail given the speed of the march. And because the column’s meteorology officer, H. B. Cook, was an enthusiastic photographer, Blanford acquired graphic images of the regions through which they passed that he then used to illustrate features of arid land erosion. The lithographs appeared in Blanford’s 1870 volume on the geology and zoology of Abyssinia in which he also used cross-sections to show general rock types visible in his area images that allowed him to generalize about relationships of strata. But what caught Blanford’s attention was the extraordinary rapid rate of erosion that occurs in desert regions where, though total rain fall is often pitifully small, yet deep, steep-sided canyons are cut by periodic torrential streams. Blanford had observed similar features earlier in northwest India, especially in the Deccan volcanics in the vicinity of the Narbada Valley and discussed in his chapter on the Deccan traps in the 1879 Manual of Geology, a volume coedited with H.B. Medlicott. These observations led Blanford to question the Lyellian generalization of gradualism, which he challenged as early as 1876 in his discussion of the arid basins of Persia, and again in 1879, as well as in his presidential address to the Geological Society of London in 1890.