10 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTAN'A. 



Although running water may in those far off times have played, 

 the chief part in determining the configuration of the hills and ridges, 

 it is certain that now and for a long time past it must have exercised a 

 very subordinate influence, and the principal agents of denudation 

 at the present time are the intense heat of the sun, or rather the 

 great alternations of temperature that affect the superficial layers of 

 the rocks during the day and night, and the violent winds that sweep 

 over the country during the greater part of the year, aided by the 

 sand they bear with them. The operations of these agents of denu- 

 dation have been fully described and explained by Dr. Johannes 

 Walther in his work " Der Denudation in Der Wiiste ", 1 illustrated 

 by many examples from the Egyptian desert. Insolation, or the action 

 of the sun's heat, is explained at page 19 of the work cited, and is 

 shown to be due to the alternate expansion and contraction of the 

 outer layers of the rock surface, while the inner layers remain at a 

 more equable temperature, and do not therefore change in volume. 

 In Rajputana the results of this action are as conspicuous as they are 

 in Egypt, and the fragments, especially those of the compact lavas, are 

 everywhere traversed by fine cracks, the " halbgesprungene kiesel " 

 of Walther, due to the same cause, so that when struck with the 

 hammer they break up into a number of small sharp -cornered frag- 

 ments, and it is often very difficult to obtain a good hand specimen. 

 The peculiar half-shattered condition of the boulders of this lava and 

 other crystalline rocks in the boulder bed of the Salt Range, which 

 renders them liable, however solid they may be in appearance, to fall 

 to pieces at a blow from a hammer, may be due to the same cause. 



Deflation, or the action of sand laden wind, is described in 

 Chapter I, section 5, and Chapter IV, section 1, of the same work. 

 The peculiar character of the rock sculpturing due to this cause has 

 been described by Mr. R. D. Oldham z who draws attention to the 

 difference between the polished surfaces and striae produced in this 



1 Konigl. Sachsischen Gesellschaft der Wissenchaften, Vol. XVI, No. Ill, 

 1S9S. 



2 Rec. Geol. Surv. hid., Vol. XXI, Pt. 4, p. 159. 



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