Lower Carboniferous Chert-Formations of Derbyshire. 159 



position and extent of these . drift- sheets could be explained by 

 regarding them as the broad terminal belts of debris concentrated 

 where the ice from the basins thinned off towards its periphery, 

 and where also its motion was checked by the rising slope of the 

 ground. The debris-choked outer margin of the ice may be 

 supposed to have become stagnant after its final forward spurt, 

 and in its waning phase most of its thaw- water probably escaped 

 backward into the basins, leaving Avide stretches of bare boulder- 

 clay unencumbered with water- washed material. Many peculiarities 

 of the drift-features were explicable on the supposition that the 

 ice-movement was not continuous and regular, but proceeded, 

 at the margin of the ice-sheets, by alternations of quick advance 

 with longer intervals of stagnation or relative quiescence, such as 

 have been observed now in existing glaciers and ice-sheets in many 

 parts of the world. 



The unequal distribution of the Glacial deposits in the area of 

 sc mty drift was then discussed, along with some local peculiarities 

 in the shape assumed by the deposits in several places, and it was 

 shown that the difference between the aspect of the main drift- 

 sheets and the scattered drift could be accounted for by the 

 difference of local conditions, which led to original irregularity of 

 deposition and to early exposure of certain tracts to exceptionally 

 vigorous erosion. It was also pointed out that the local incidence 

 of giaciation may often have been an important factor, as it is 

 known from existing conditions in Arctic lands that great stretches 

 of moving ice may leave bare land, aside from its pa^th, at lower 

 levels on its flanks. 



Comment was then made on the curious rarity of peat or other 

 land-detritus in boulder-clay known to have been derived entirely 

 from the land, and this was thought to indicate that the condi- 

 tions for a long period before the actual giaciation had been 

 unfavourable for the growth of timber or peat-producing vege- 

 tation. 



A brief review was given of the minor changes and new erosion- 

 features produced in the hills as a result of the giaciation. The 

 effect of Postglacial erosion and deposition in modifying the 

 Glacial features was also referred to. 



February 25th.— Mr. R. D. Oldham, F.K.S., President, 

 in the Chair. 



The following communication was read : — 



' The Lower Carboniferous Chert-Formations of Derbyshire.' 

 By Henry Crunden Sargent, F.Gr.S. 



The chert-formations occurring in the Carboniferous Limestone 

 and associated rocks of Derbyshire ma} r be classified under two 

 heads : — 



(1) Those which owe their silica to gaseous or aqueous emanations from 



igneous rocks. 



(2) Those which derived their silica from the land by moans o( ohemioa] 



denudation. 



