68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY". 



endeavouring to account for the fractured and crushed condition of 

 the rocks under these boulders by precipitation from floating ice, 

 he gave an account of his discoveries on the high limestone plateau 

 north-east of Clapham (Yorkshire), where there is a " ghastly 

 array " of many hundreds of large Silurian grit and slate boulders, 

 nearly black in colour. From many facts and considerations the 

 author endeavoured to show that most of the pedestals of these 

 boulders must have existed before the arrival of the boulders, while 

 the pedestals acquired through the boulders protecting the under- 

 lying rock from denudation were generally imperfectly formed. 

 On the Clapham plateau he found that the average vertical extent 

 of denudation around the boulders with acquired pedestals was not 

 more than on the Eglwyseg plateau, or about six inches. In the 

 case of boulders which were not well adapted to concentrate rain- 

 water, the extent of lowering of the surrounding rock-surface was 

 often inappreciable ; and this accounted for the continuous extension 

 of flat limestone rock-surfaces under some of the boulders. The 

 author then described what he had found to be preglacial as well 

 as postglacial rain -grooves on limestone rock-surfaces, near Minera 

 and on Halkin mountain (North Wales), where he found the average 

 depth of those of the grooves which were probably postglacial to 

 be about six inches. In conclusion the author entered into a con- 

 sideration of the time which has elapsed since the close of the 

 glacial period, and stated the main results of his observations as 

 follows : — 



1. That the average vertical extent of the denudation of limestone 

 rocks around boulders has not been more than six inches. 



2. That the average rate of the denudation has not been less than 

 one inch in a thousand years. 



3. That a period of not more than six thousand years has elapsed 

 since the boulders were left in their present positions by land-ice, 

 floating ice, or both. 



Discussion. 



Prof. Hughes bore testimony to the value of the labours of Mr. 

 Mackintosh in recording the character and manner of occurrence of 

 boulders scattered over Wales and the North of England. He 

 pointed out, however, that the calculations of the rate of waste 

 made by the authors quoted was founded upon observations over a 

 very large area, where the hard and soft rocks compensated for one 

 another, and an average was obtained, but that this rate could not 

 be taken as that of any one rock-surface such as referred to by 

 Mr. Mackintosh. He had himself, in 1867 *, described the Norber 

 boulders, and proposed the question, "Assuming the average periodic 

 rainfall to have been constant, or any rate to be determinable, and 

 the quantity of limestone removed by a given quantity of rain- 

 water to be known, to find how many years have elapsed since this 

 limestone was exposed to subaerial denudation." But the rainfall 



* Geol. and Polytechnic Soc. West Riding, Yorkshire, July 1867, p. 11. 



