[ 220 ] 



XXYI. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



[Continued from p. 79.] 



June 23, 1886.— Prof. J. W. Judd, F.R.S., President, in the Chair. 



rPHE following communications were read : — 



-*- 1. "On some Perched Blocks and associated Phenomena." 



By Prof. T. M c Kenny Hughes, M.A., E.G.S. 



The Author described certain groups of boulders which occurred 

 on pedestals of limestone rising from 3 to 18 inches above the level 

 of the surrounding rock. The surfaces of these pedestals were stria- 

 ted in the direction of the main ice-flow of the district, while the 

 surrounding lower rock in no case bore traces of glaciation, but showed 

 what is known as a weathered surface. 



He inferred that the pedestals were portions of the rock protected 

 by the overhanging boulder from the down-pouring rain, which had 

 removed the surrounding exposed parts of the surface. When the 

 pedestals attained a certain height relatively to the surrounding rock 

 the rain would beat in under the boulder, and thus there was a 

 natural limit to their possible height. 



He referred to the action of vegetation in assisting the decompo- 

 sition of the limestone, and considered that there were so many 

 causes of different rates of waste and so many sources of error, that 

 he distrusted any numerical estimate of the time during which the 

 surrounding limestone had been exposed to denudation. 



Considering the mode of transport of the boulders, he thought that 

 they could not have been carried by marine currents and coast-ice, 

 as they had all travelled, in the direction of the furrows on the rock 

 below them, from the parent rock on the north. Moreover, marine 

 currents would have destroyed the glaciation of the rock and filled 

 the hollows with debris. 



Furthermore, the boulders and striae are found in the same district 

 at such very different levels and in such positions as to preclude the 

 possibility of their being due to icebergs. 



Nor could the boulders represent the remainder of a mass of 

 drift which had been removed by denudation, for the following 

 reasons : — 



1. They were all composed of one rock, and that invariably a 

 rock to be found in place close by. 



2. Any denudation which could have removed the clay and 

 smaller stones of the drift would have obliterated the traces of 

 glaciation on the surface of the rock. 



3. The boulder which had protected the fine glacial markings 

 below it from the action of the rains would certainly in some cases 

 have preserved a portion of the stiff Boulder- clay. 



4. The margin of the Boulder-clay along the flanks of Ingle - 

 borough was generally marked by lines of swallow-holes, into which 



