580 TRANSACTIONS OF SECTION G. 



far down as ' Halfway.' The author has traced them on the flanks of the Brecon 

 Beacon as high as Newadd (886 feet) and down to Talybont, where a large one 

 was found close to the canal tunnel (400 feet). At Llaugorse they form part of 

 a moraine which dams back the drainage to form Llangorse Lake. They are 

 found in large numbers at Talgarth, and were traced up Cwm Pwll-y-wrach as 

 high as the 800-foot contour. Numbers of smaller boulders were found mixed with 

 Old Red Sandstone material in gravel deposits near Three Cocks Junction, a little 

 stream revealing good sections in mounds of rearranged and roughly bedded drift 

 deposits. No trace of these foreigners was found in the valley of the Wye from 

 Builth Wells to Three Cocks, nor were any found in fine sections of boulder-clay 

 examined at Llandrindod Wells. 



The author hopes to continue the investigation of these deposits, but believes 

 that sufficient evidence has been collected to point to a local glacier at first in each 

 of the valleys of the Usk and the Wye. The Usk glacier was fed from the 

 Carmarthen and Brecon Fans, but appears to have been overridden subsequently' 

 by a stream of foreign ice from the direction of Llandovery, bringing the brecciated 

 erratics and pressing down the valley to Llangorse, Talgarth and Three Cocks. 

 It is to the pressure of this foreign ice that he would attribute the overflow of the 

 Old Red Sandstone drift by the Cray Valley, on to the Carboniferous rocks of 

 Penwyllt, and up Dyrt"ryn Crawnon and through the faulted gap of Nant Trehl into 

 the Rhymney and Sirhowy valleys (as reported by the Geological Survey). 

 Among the erratics of the Wye Valley were tough green grits, which were subse- 

 quently found quarried at Builth, but marked on the geological map as ' Green- 

 stone.' Several interesting stream diversions, owing to accumulations of morainic 

 material, were observed. Amongst the more important were the diversion of the 

 Usk from a wide valley to a narrow gorge at Aberyscir by a moraine at Cradog ; 

 of theHonddu at ' The Forge,' Brecon, to the glacial gorge which runs below the 

 Priory Church ; and the reversal of the drainage of the Afon Houddu and Olchon 

 Brook at Llanvihaugel and Pandy by the morainic gravels which block the wide 

 valley between Bryn-aro and Skirrid-fawr, down which these streams no doubt 

 flowed in pre-glacial times to join the L'sk at Abergavenny, whereas now they 

 have been diverted into the Monnow, and so reach the Wye at JMonmouth. 



Only one case of a dry valley which had been a glacier-lake overflow was 

 noted, and that was the little gorge called Cwm Coed-y-cerig, by which the 

 drainage of Grwj'ne Fawr appears to have been carried oft' when the lower part of 

 its present valley was obstructed by a lobe of ice from Crickhowell, but it was not 

 cut deeply enough to continue to take the stream when the lobe was withdrawn. 



3. A Silurian Inlier in the Eastern Mendips, 

 By Professor S. H. Reynolds. 



The rocks described are exposed along the crest of the Mendip range from 

 Beacon Hill on the west to near Downhead on the east, a distance of between two 

 and three miles. They consist of tuffs of several types, and of andesitic lavas. 

 Tlie exposures are everywhere surrounded by the Old Red Sandstone, and till 

 recently it seemed probable that the igneous series was of Old Red Sandstone age. 

 About twenty species of Silurian fossils have, however, now been met with in the 

 tuffs underlying the andesitic lava, and show that the rock is either of Weulock 

 or Llandovery age. In addition to the normal tuffs there are several exposures of 

 a remarkable rock consisting as a rule of well-roimded blocks of andesite, reaching 

 a maximum length of 2 feet, embedded in a typical ashy matrix. The relations 

 of this rock to the others are not yet clear. 



Hitherto contemporaneous volcanic rocks of Silurian age have only been 

 clearly recognised at two localities in the British Isles, viz., Clogher Head in 

 Co. Kerry, and Tortworth in Gloucestershire ; the Eastern Mendips constitute a 

 third area. 



