TRANSACTIONS OF SECTION C. 579 



6. Further Note on the Occurrence of Diamonds in the Matrix in New 

 South Wales} By Professor T. W. Edge^vorth David, F.R.S. 



7. Report on the Correlation and Aye of South African Strata, 



8. Report on Geological Photographs . 



9. Recent Observations at Vesuvius. Iti/ Dr. H. J. Juuxston-Lavis. 



WEDNESDAY, AUGUST 8. 



The foUowiDg Papers and Report were read : — 



1. A Criterion of the Glacial Erosion, of Lake- Basins. 

 By R. D. Oldham. 



Apart from the question of whether lake-basins can be eroded by glaciuis, a 

 criterion by which the possibility of such an origin can be tested is desirable. 

 Such a criterion appears to exist in the fact that, given a glacier and a rock-basiu 

 in its bed, one of two things must happen : either the glacier will How, as a whole, 

 through the basin, or the ice lying in the depression will remain stagnant while the 

 upper layers flow over it. The first must be the case if the rock-basin has been 

 scooped out by the glacier, and a cross section of the valley and lake will show a 

 continuity of the above and under water slopes, with no interruption at or near 

 water-level. In the second case the rock-basin must have originated independently 

 of the glacier, and a terrace will be formed at the base of the moving portion of 

 the glacier, this terrace being partly due to the wearing away of the sides of the 

 valley, partly to deposition of moraine matter brought down by the glacier. The 

 deeper of the Scottish lochs, when tested by the survey being carried on by Sir J. 

 Mm-ray and Mr. Lawrence PuUan, show just such a terrace at or a little below water- 

 level, and this terrace appears to occui- constantly in places where the possibility of 

 its having been formed subsequently to the retreat of the glaciers is excluded. If 

 this terrace is of glacial age it would prove that the lake-basins were not scooped 

 out by glaciers, but owe their origin to some other cause. 



2. Notes on the Glaciation of the Usk and Wye Valleys. 

 By Rev. W. Lower Caktek, M.A. 



During a recent holiday the author was able to study the glacial deposits 

 of the district to the north of the South AVales coalfield. The gravelly deposits 

 of Old lied Sandstone material which are characteristic of the valley of t]^e Usk 

 between Brecon and Abergavenny (see ' Geological Su rvey Memoir') have been 

 traced for some distance to the north-east of Brecon and up the valley of the Usk 

 as far as Trecastle. Here the river breaks away from the old ' through ' valley, 

 which is continuous to Llandovery and rises in the Carmarthen Fans to the south. 

 On the top of this red drift were found large numbers of erratics of volcanic ash 

 and brecciated limestone, which the author supposes to have been derived from 

 Ordovician outcrops to the west or north-west of the area in question. These blocks, 

 which run up to two tons or more in weight, are found all down the Usk Valley 

 below Trecastle, and over the col towards Llandovery, in the Gwyddrig Valley, a» 



' Included in Abstract, p. 562. 



P p2 



