Geological Society. 567. 
sited are replete with dislocations, and penetrated at many points by 
ridges of trap rock, it is to be inferred, that during and after the 
evolution of this volcanic matter, great and successive elevations of 
the bottom of the sea took place, throwing up the drifts to the various 
heights at which we now find them. As soon as the land was raised 
from beneath the sea, the present rivers, it is conceived, began to flow ; 
passing through the ridges by gorges produced by great lateral cracks 
the result of elevation ; and that these streams have since merely trans- 
ported to short distances those broken materials which were previous- 
ly gathered together by subaqueous drift. To prove that the drifted 
matter of each district within this region may be referred to disturb- 
ances purely local, it is shown that although wherever the hills have 
been elevated from N.E. to S.W. the lines of drift are from N.W. to 
S.E., yet in those contiguous tracts which have been elevated in other 
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 under 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 
by 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 andthe 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. 
