2l6 EXPLORATIONS IN TURKESTAN. 
heif^lit. Where lakes are found at the lower euds 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 indi- 
cation of a lar<,'er munber, and it is entirely possible that further obser\-ation will 
show that the lakes clianged 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 es.sential point in our study of the recent geological histor)' 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 con- 
clusion, namely, that during the Quaternary- era there have been a number of colder 
or glacial epochs, five or more, separated by wanner 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 continent';, it becomes 
in a ver^' high degree probable. Much confidence is therefore felt in the theon,- 
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. 
