1853.] TRIMMER ON THE ERRATIC TERTIARIES. 283 



treated of in another paper. On the West they have a greater 

 southern extension than has been assigned to them by high autho- 

 rities. T have myself traced the Boulder-clay, with fragments of 

 granite, Antrim chalk, and shells, nearly to St. David's Head ; and, 

 in travelling across the country, I have observed the rolled gravel of 

 the Upper Erratics extending to the neighbourhood of Milford. In 

 Ireland, the Ordnance brickworks at Youghal, in the county of Cork, 

 were established on boulder-clay. If to this be added the well- 

 known existence of grooved and scratched rocks at the southern 

 extremity of the county of Kerry, it will be impossible to exclude 

 any considerable area in Ireland or South Wales from the glacio- 

 mariue operations ; and the boundaries which have been assigned to 

 the boreal ocean, in order to explain the geological relations of the 

 existing fauna and flora of the British Isles, will require considerable 

 modification. 



In the Valley of the Severn, the most southern locality yet re- 

 corded for shells and granitic detritus is about three miles south of 

 Worcester*. 



Sir Roderick Murchison has stated, that though boulders of 

 northern origin diminish in quantity south of Bridgnorth and Wol- 

 verhampton, coarse gravel, composed of the same materials, is pro- 

 longed, like the tail of a delta, through Worcestershire, until it dies 

 away in the fine gravel and silt of the Vale of Gloucester f. It is an 

 important question in the history of these deposits, whether this 

 dying-off arose from an original cessation, at that point, of the 

 agencies which transported the northern detritus ; or whether it is 

 the result of denudation and re-arrangement. I hold the latter opi- 

 nion, because it accords with the phsenomena of denudation which I 

 have observed along the main lines of drainage in South Wales, and 

 which appear to be repeated on a larger scale in the Valley of the 

 Severn. In such valleys the boulder-clay is rarely found, except near 

 their upper portions and near the limits of that deposit as regards 

 elevation above the sea. In their lower regions it occurs only as 

 small broken patches ; and extensive areas are either wholly free 

 from the presence of the erratic deposits, or exhibit them only under 

 their reconstructed form of rolled gravel at low levels. At the same 

 levels, however, adjoining minor valleys are filled with boulder-clay, 

 and large spaces between the great lines of drainage are covered by 

 the sand and gravel of the upper ei'ratics resting on boulder-clay. 

 This would be very evident if I could have an opportunity of ex- 

 hibiting the lost map of the Surface Geology of part of Cardigan- 

 shire and Pembrokeshire which I constructed for the Government 

 Geological Survey, and to which I have so often referred. 



In Ratter's 'Delineation of Somersetshire,' published in 1829, there 

 is a sketch of the geology of the county which appears to be chiefly 

 a compilation from various sources ; but, as the authorities are not 

 always cited, it is difficult to separate the compiled from the original 



* Pioc. Geol. Soc. vol. ii. p. 95 ; and Sil. Syst. p. 534. 

 t Proc. Geo!. Soc. vol. ii. p. 333. 



