298 Geological Society. 



natural time-measurers by their protecting the rock-surface under- 

 neath from the action of rain, which, around the boulders, denudes 

 the surface, especially on the leeward and windward sides, where 

 hollows resulting from pluvio-torrential action may generally be 

 seen. He then described and explained the origin of the different 

 forms of supports uuder boulders, which graduate from flat surfaces 

 to pedestals of various forms, which he divided into appropriated 

 (or preexisting), and those acquired through the boulders protect- 

 ing the underlying rock from denudation. The author then de- 

 scribed the positions of boulders on the high and uninhabited 

 Eglwyseg limestone plateau near Llangollen, where it is certain 

 they had never been disturbed by man. There he found that the 

 average vertical extent of denudation by pluvial action around the 

 boulders, since their arrival, was not more than six inches. After 

 endeavouring to account for the fractured and crushed condition of 

 the rocks under these bo alders 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. Prom 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. 



3. " Notes on the Corals and Bryozoans of the Wenlock Shales 

 (Mr. Maw's Washings)." By G. E'. Vine, Esq. 



