height. Where lakes are found at the lower ends of rivers, they show changes of level that are only explicable on the theory of climatic oscillations of decreasing intensity. Only two of these have been clearly identified, but there is some indication of a larger number, and it is entirely possible that further observation will show that the lakes changed as often as the glaciers and the rivers. When the glaciers advanced and built their moraines, the rivers swung laterally, aggrading and broadening their valleys, and the lakes expanded and spread their silts. When the glaciers retired the streams cut gorges and the lakes contracted.
The essential point in our study of the recent geological history of Turkestan is this : From three separate lines of reasoning, based on the allied yet distinct phenomena of glaciation, terracing, and lake expansion, we arrive at the same conclusion, namely, that during the Quaternary era there have been a number of colder or glacial epochs, five or more, separated by warmer interglacial epochs when the climate was similar to that of to-day ; and further, that these epochs progressively decreased in length and intensity.
When a single theory fits all the facts of a single series of phenomena, it becomes probable ; when it fits the facts of three distinct series of phenomena, it becomes highly probable ; and when it fits the facts of several continents, it becomes in a very high degree probable. Much confidence is therefore felt in the theory above announced. It is yet to be applied to the basins of the Caspian and Aral seas on the west. A most interesting additional step would be to see if the theory is capable of explaining the great basin deposits of Central Asia which lie to the east of the region here described.