GENERAL KELATIONS OF ROCK GROUPS. 29 



a distance; they are of limestone and hard calcareous shale from 

 the Vindhyans, often of considerable size^, distinctly and unmistakably 

 roUedj and all the rocks in the neighbourhood^ including those on 

 which the Talchirs rest, are metamorphic. The difficulty remains. 

 Any current or wave action which could move the boulders to the 

 spotj would sweep away the fine mud ia which they are imbedded. 



General Portlock, President of the Geological Society in 1857,* 

 whUe mentioning my view, expressed his preference for Mr. Mallet's 

 theory of mud sHps on a coast. This theory, which was proposed in 

 order to explain the phenomena of the glacial drift and the grooves 

 and scratches so prevalent on some of the rocks of Great Britain and 

 Irelandj Northern Europe, and parts of America, was, that both the 

 transport of blocks to great distances, together with gravel, sand, and 

 mud, and the grooving of rock surfaces, may be effected by the slow 

 movement of masses of detritus under water, and their " slippage" on 

 the inclined rock surfaces of the sea bottom.f Mr. Mallet considered 

 that the mere weight of loosely aggregated materials would suffice to 

 carry them seaward even on a very low slope. Of course this view, if 

 Mr. Mallet's premises be granted, would be applicable to deposits 

 formed in lakes, and perhaps under peculiar circumstances in river 

 valleys. 



I cannot give in my adherence to the priaciple, notwithstanding 

 its ingenuity. That slipping to some extent takes place, that fine 

 materials at all events are moved seaward to a depth equal to the 

 influence of tidal currents, is highly probable, if not certain ; but that 

 anything like the extensive and general movements of loose detritus 

 suggested by Mr. Mallet is of common occurrence, has certainly not 

 yet been proved, so far as I am aware. The evidence obtained by the 



* Q. J. G. S., vol. xiii, p. cxxiii. 



t Journal of the Geol. Soc. of Dublin, 1852, vol. v, pp. 121—129. 



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