GEOLOGICAL DIVISIONS. 20p 



been correct, and this rock may indicate that the great eruptions 

 which in recent and sub-recent times culminated in the building up 

 of the gigantic cones of Eastern Persia had already commenced in 

 Siwalik times. This rock, together with some curious basic dykes 

 of very recent aspect found in the same neighbourhood, will be 

 again referred to in Part II of this Memoir (see page 88). 



Section 5.— Recent and sub-recent deposits. 



Under this heading come all the rocks which have been depos- 

 ited since the termination of the upheaval of the ranges. Their 

 principal characteristics have been mentioned when the hydrography 

 of the district was described. There it was mentioned how they 

 have filled with conglomerates and sands the great depressions 

 that lie between the groups of mountains ; how they cover with a 

 horizontal layer of silt the plains that once were lakes ; how they 

 have accumulated to form the gigantic talus or "daman" skirting the 

 mountains. 



In a country where the rainfall is so scanty and irregular, where 

 the hydrography is in quite a rudimentary condition, it naturally 

 follows that we cannot expect to find the section of the plain to 

 exhibit those beautifully regular curves which any river or valley 

 shows when followed from mountain to sea-coast or lake basin in 

 a region where the rainfall is normal. Undulations barely per- 

 ceptible to the eye in such alluvial plains when plotted on an 

 exaggerated vertical scale are found to follow very definite laws, 

 taking the shape of a parabolic curve concave towards the sky, 

 The desert plain which, when seen from a height, also appears quite 

 flat, does exhibit shallow undulations leading to well-marked differ- 

 ences of level ; but these instead of obeying definite laws are dis- 

 tributed quite irregularly. The topographical survey of this desert 

 has not been carried to that degree of minute detail which would 

 allow this to be shown by representations of actual sections of the 

 ground ; but it is strikingly illustrated by the juxtaposition of 

 numerous independent basins of drainage, often at well-marked 



( 3« ) 



