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directions the course of the drift changes immediately with the varia- 

 tion of the strike. Thus on the exterior margin of the great coal-field 

 of S. Wales vast quantities of materials resulting from the breaking 

 up of the carboniferous series have been dispersed to the N.E., N., and 

 N. West, directions excentric from the broken margin of that elevated 

 tract. 



In Pembrokeshire, again, where the prevalent lines of strike are 

 from E. to W., the drift has been carried southwards. Conceiving 

 that the great masses of these drifts have been formed at various pe- 

 riods mider the sea, either in gulfs, estuaries, or straits, and have been 

 raised up at different periods when the solid strata were elevated, 

 the author then proceeds to consider the probable conditions of the 

 surface of this portion of the country for some time after such emer- 

 sion, and yet at a period comparatively remote. He instances many 

 flat embayed tracts which, from the equable surface of the sand and 

 gravel, are supposed to have been for some time under water, occu- 

 pying lakes which have been drained by the deepening of gorges is- 

 suing from these bays ; since it is shown that a very slight difference 

 of level in the beds of rivers at several gorges would effectually bar up 

 the present streams, and pond them back into lacustrine expanses. 

 Hence he infers that slight additional movements of the land, aided 

 by the excavating process of the rivers themselves, may have operated 

 in draining these flat tracts. A large part of Herefordshire watered 

 bv the Wye is supposed to have been under such waters, which have 

 since escaped by the picturesque gorges of Ross and Chepstow. The 

 Vale of Radnor is a similar case ; now drained only by a feeble rivulet. 

 But the clearest examples of successive lacustrine expanses are ex- 

 hibited in the descent of the Teme ; first in the tract still called 

 <' Wigmore Lake;" from whence the superabundant waters have 

 escaped through the upper Silurian rocks in the gorge of Downton 

 on the rock ; and next in various expansions and contractions between 

 Ludlow and the Abberley Hills, where they have been again barred up 

 by that ridge until the gorge at Knightwick Bridge was deepened, 

 opening out a channel for their escape into the great Valley of the 

 Severn. The finely levigated sand, marl, and mud, at small heights 

 above the present stream point to this anterior lacustrine condition. 

 The period of the final desiccation of these river-lakes, and the reduc- 

 tion of the rivers to their present channel, is supposed to have been 

 contemporaneous with that recent elevation which in raising the land 

 to greater heights brought up large adjacent portions of the bottom of 

 the sea, and to the consideration of which the second part of the me- 

 moir is devoted under the head of " northern drift." 



In the region of Welsh and local drift attention is specially called 

 to the length of time during which existing causes have been in un- 

 disturbed action, as proved by the magnificent mass of Travertino 

 formed and still forming at the Southstone rock; whilst he also points 

 to the discovery of shell marl in a bog near Montgomery, containing 

 several species of Lymncea, which has evidently been formed in the 

 manner described by Mr. Lyell in his memoir on the marl loch of 

 Forfarshire. 



