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Imperial Gazetteer of India, v. 1, p. 51.

GEOLOG Y 51 open valleys, with rivers near their base-level of erosion, and the gently undulating plains are due to the toning down of the rocks by long exposure to the weather. A very different state of things is disclosed in the land lying The folded to the west, north, and east of the great Indo-Gangetic alluvial extra- peninsular belt: in Sind, Baluchistan, the Himalayan belt, Assam, and area. Burma we have abundant evidence of repeated immersions beneath the ocean. In this area the directions of the mountain chains are determined by comparatively young rock-folds, while the region having been but lately elevated, its rivers are swift and torrential, cutting down their beds so rapidly that the valley sides are steep, with loosened material, always ready to slide off in destructive landslips. In attempting to express these two distinct geological stories Correla- in European terminology we find that our simplest and most tion of easily translated characters are preserved in the marine fossili- strata with ferous strata. while it is practically impossible to correlate the European directly the land and fresh-water formations which are so largely scale. developed on the Peninsula with their equivalent stages in the European standard scale. The reasons for this contrast are simple. Conditions of life Variable rate of are much more uniform, and facilities for migration much evolution more perfect, in the ocean than on land. On land areas there in isolated is a greater variety of physical features, and a greater diversity, land areas. therefore, of climate and other conditions which affect the distribution of living beings. Consequently, when such areas are cut off from one another by impassable physical barriers, the intermingling of plants and animals is prevented, and evolution proceeds at independent rates in the separated areas, attaining corresponding stages at quite different times. As an example of the errors which would arise if we compared the fresh-water and land fossils of widely separate areas with one another, we have, in the existing indigenous mammalian fauna of the isolated Australian continent, a stage of evolution about equivalent to that which characterized Europe in Jurassic times. This want of correspondence during the same period of living forms in widely-separated land areas is one of great importance to the Indian geologist, who has had the point most strikingly brought home to him in his attempt to deter- mine the age of the great coal-bearing system in India. The luxuriant growths of ferns, horsetails, cycads, and conifers which flourished in the great river-valleys of the old Gondwana continent did not make their appearance in Europe until well on in the Mesozoic era, yet, from other evidence, we know E2