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Memoir on the Geology of the 



[Oct. 



erosions (April 27, 1S36), and again two months afterwards, the peb- 

 bles were all at rest ; but, during the monsoon, the impetus of the 

 torrent must put them all in motion by its rapidity and volume, 

 particularly those in the middle within influence of the impetuo- 

 sity of the waters, malting them whirl round violently ; and so, by 

 constant and violent attrition, the cavity must enlarge and the masses 

 themselves get rounded and smaller; the possibility of their getting 

 out diminishing, as the depth of the cavity increases. If by chance 

 a slight depression existed on the surface of the rock, and the running 

 water, either from impediments in its course, or from its comparative 

 rest on passing over these nearly horizontal masses, should form a 

 whirlpool, and if this should happen to be above the little depres- 

 sion, where sand and pebbles may have accumulated, these must be 

 put into whirling motion, and begin the work of erosion. 



Nearly all the pebbles and masses in these cavities are rounded, some 

 being of an oval shape ; their weight varies from hundreds of pounds 

 to a few ounces. Now, the very large masses could not possibly have 

 been rounded by rolling down so short a distance as three miles and a 

 half, from the source of the river ; and, although we see some of these 

 large, rounded and polished masses, in its bed, both above and below 

 this spot, we shall shew hereafter, that these cavities are found higher 

 lip and lower down likewise ; and there is nothing improbable in sup- 

 posing that the volume and the impetus of the stream may be such at 

 times, as to force some of these stones out of the perforations, and 

 leave them in some corner of the bed of the river. I do not mean to 

 contend that, after rolling many miles, the angular masses become 

 rounded ; but what I maintain is (and it is the opinion of all those who 

 have seen such erosions in India), that they are formed by the long 

 continued tossing of the rounded blocks within them ; and that these 

 last did not by accident fall in, ready rounded by long rolling from a 

 great distance ; and that they are the original cause of these cavities, 

 and produce their increment. 



The above explanation supposes a certain cavity to exist, ready 

 made, on the surface of the rock, and then the rest of the process is 

 easily understood ; but the question is, what produces those cavities in 

 the first instance. The answer is easy enough — they are the holes 

 left by the nests and oblong pieces of granular hornblende, which are 

 found abundantly in the rock. "We know that if cavities exist at the 

 bottom of pools, with comparatively smooth water, through which an 

 impetuous torrent rolls its waters, whirlpools are formed. But in the 

 incipient state of the cavity, the fall of the water on them is sufficient 

 to set the pebbles in agitation. From what we have detailed, we may 

 safely assert that these cavities are formed and increased by the attri- 

 tion of these rounded stones, which act with more violence on the sides 



