234 Reports & Proceedings—Geological Society of London. 
course of crystallization from a complex magmatic system the mode 
in which the effective pressure varies must be considered as well as 
the mode of cooling, for change of pressure may affect the order of 
erystallization. 
REPORTS AND PROCHEHDINGS. 
I.—Grotocicat Socrery or Lonpon. 
1. February 23, 1916.—Dr. Alfred Harker, F.R.S., President, 
in the Chair. 
The following communication was read :— 
‘‘On the Origin of some River-Gorges in Cornwall and Devon.” 
By Henry Dewey, F.G.S. (Communicated by permission of the 
Director of H.M. Geological Survey.) 
In North Cornwall, near Tintagel, there is an area of peculiar 
topography characterized by the presence of an upland plain or 
plateau. The plateau is dissected by deep gorges, with their walls 
scarred by potholes through which the rivers flow in a series of 
waterfalls, cascades, and rapids. aux 
This plateau is terminated inland by degraded cliffs rising abruptly 
from 400 feet above sea-level, while the plain slopes gently to the 
recent sea-cliffs, mostly over 300 feet high. The plateau has been 
cut across rocks of different degrees of hardness, and is overlain by 
deposits of detritus and peat. Wherever the plain occurs the 
scenery is featureless, and the land boggy and waterlogged. 
The widespread occurrence of this plain over Cornwall and Devon 
at a uniform height suggests that in its final stages it was a plain of 
marine erosion. ‘he author accepts Mr. Clement Reid’s conclusion 
that its date is not later than Pliocene. Its uplift in post-Plocene 
times led to rejuvenescence of the rivers, initiation of coastal cascades, 
and the production of gorges aided by the formation of potholes. 
At Lydford, on the western flank of Dartmoor, the uplift led to 
the diversion of the Lyd by a stream that breached the valley-side 
and tapped the head-waters of the river. 
A small stream, the Burn, now flows past Was Tor and Brentor, 
through the valley formerly occupied by the Lyd. The shortened 
journey to the lowlands bestowed such enhanced cutting-power upon 
the river that it quickly incised a chasm through which it now flows 
more than 200 feet below the base of its former valley; while 
a tributary enters as a waterfall from a hanging valley near Lydford 
Junction. 
The elevation of the land also led to formation of gorges of similar 
character in other upland plateaux. These plateaux have been 
described by Mr. Barrow in the Quarterly Journal of the Geological 
Society, and reference is made to them by the author in connexion 
with the effects upon them of the uplift. 
There are thus in Cornwall and Devon two characteristic types of 
scenery, to which in great part these counties owe their charm. Wide 
featureless plains covered with heath and marshland and dominated 
by tors and crags, on which the drainage is sluggish and vague, 
a 
