86 T. BELT ON THE DEIFT OF DEVON AND CORNWALL. 



ice from the north could have scored the eastern sides of the Ameri- 

 can hills, nearly at right angles to the slope of the continent, or 

 have been forced up the valley of the St. Lawrence, before the ocean- 

 depression was filled, seems as improbable as that it should flow 

 along the ridges of the Alps, and not down the valleys. Doubtless 

 the culmination of the Glacial Period was preceded and followed by 

 both local and continental ice systems ; but, in addition to these 

 agencies, there is required, to explain the grander phenomena, the 

 action of circumpolar ice filling the northern part of the Atlantic 

 basin down to about latitude 39° on the American side, and about 

 latitude 40° on the European side. 



The main feature of the Glacial theory that I advocate is, that 

 the glaeiation of North-eastern America and North-western Europe 

 was principally due, not to local and continental ice, but to that 

 filling the northern part of the Atlantic basin, and blocking up the 

 drainage of the continents as far as it extended. The theory re- 

 quires no interglacial periods, no oscillations of the earth's surface ; 

 yet I know not of any fact in the composition and distribution of 

 the glacial beds on either side of the Atlantic that does not find in it 

 a natural and easy interpretation. 



4. Correspondence with the drift of the south-east op 

 England. — It is clear that, if this theory is correct, evidence of the 

 existence of the great lake and of its sudden and torrential discharge 

 ought to be found over the whole of the vast area its waters once 

 covered. I have found this evidence over much of Northern Europe, 

 and in our own country, up the valley of the Severn, and over the 

 whole of the eastern coast : but in this paper I confine myself to the 

 south of England, as I thus avoid questions connected with the 

 former presence of land ice, and with what many geologists consider 

 evidence of marine submergence. I shall pass rapidly in review 

 the surface geology of the rest of the southern counties. 



Dorsetshire. — Upland deposits have been described on the Black 

 Downs, to the S.W. of Dorchester, at Preston and Osmington, and 

 in the Admiralty quarries on the top of the Isle of Portland*. The 

 latter are remarkable, as they overlie remains of Elephas primi- 

 genius and E. antiquus. The raised beach with marine shells at 

 Portland Bill is covered with the Lowland deposits, which appear to 

 have been suddenly and tumultuously produced. Prof. Prestwich 

 believes they were formed by the transient passage of a body of 

 water sweeping the land detritus down to the shore-line, and to a 

 certain distance beyond it — a conclusion identical with that arrived 

 at by Sedgwick and De la Beche with regard to the Lowland deposits 

 of Cornwall. 



Hants and Wilts. — The superficial deposits of these counties have 

 been admirably described by Mr. Codringtonf. The gravels are 

 not'eonfined to the valleys, but cover the plateaux between them up to a 

 height of 420 feet above the sea, and extend in width for a distance 

 of 20 miles. At Milford Hill, near Salisbury, Upland gravels from 



* Prof. Prestwich, Quart. Journ. Geol. Spc. vol. xxxi. p. 29. 

 t Quart. Journ. Geol. Soc. vol. xxvi. p. 528. 



