﻿64 



ME. H. DEWEY ON THE OKIGIjST OF SOME [vol. lxxii,. 



higher land beyond it ; and, although south of this point much 

 of the land rapidly falls, the feature is seen in places both near 

 the coast and inland. 



Most of this area is drained by the rivers Camel and Allen, both 

 flowing from north to south in courses roughly parallel to one 

 another and to the coast ; but they do not drain all the district, as 

 they are separated by a ridge of high ground from a coastal area 

 drained by streams flowing westwards into the sea. These streams 

 are short and formerly united with another river- valley, now buried 

 beneath the sea, and are thus remnants of a larger drainage-system. 

 One of these valleys runs parallel with the present shore-line, its 

 left-hand slopes forming the sea-cliffs, which are breached by the 

 encroachment of the sea at several points, while its right side 

 is continuous (see PI. V). Similar sea-breached river- valleys in 

 North Devon are described by Dr. E. A. N. Arber, 1 and others 

 occur in Cornwall and South Devon. 



Elsewhere in Cornwall and Devon the upland plain dominates 

 the landscape over wide areas, and more especially near Newquay, 

 on St. Austell Moors, by St. Agnes Beacon, near Camborne, and 

 also in the Land's End and the Lizard. It is conspicuous in 

 Central Cornwall, about Roche and Lostwithiel, and extends along 

 the Tamar Valley towards Launceston. 



In South Devon it truncates nearly all the land south of Dart- 

 moor, and has had important effects upon the drainage of that 

 country. East of Dartmoor it is seen especially well about Bovey 

 Tracey, Avhere it has cut into deposits assigned by Clement Reicl 2 

 to the Oligocene Period. 



Into the question of the age of this plain I do not intend to 

 enter. Eeid adduced evidence to show that the deposits resting 

 upon it are early Pliocene, and I believe most authors agree that 

 they were deposited during some part of the Pliocene Period ; but 

 the plain itself may be of much earlier age. 



The further question as to the nature of the agency which cut 

 this plain has received the attention of several investigators ; 

 but there can be no doubt that the final agent which effected the 

 planation was the sea, as is indicated by the widespread occurrence 

 of the plain, everywhere at the same altitude. 



The deposits resting upon it have long been known at St. Agnes, 

 St. Erfch, and at the Lizard. They vary from clay to gravel, but, 

 except at St. Erth, do not contain fossils. The character and the 

 depth of the deposits resting on the plain in North Cornwall were 

 tested b} r some pits dug by the officers of the Geological Survey 

 near Tintagel, which exposed about 12 feet of angular detritus over- 

 lying a bed of pebbles of vein-quartz. It is said by local observers 

 that sand containing fossil shells was found when a trench was 

 dug for the water-main many years ago. These sands, however, 



1 ' The Coast Scenery of North Devon ' 1911, pp. 230-39. 



2 ' Geology of the Country around Newton Abbot' Mem. Geol. Surv. 1913, 

 pp. 104-117. 



