Paleo-Geological and Geographical Maps of the British Islands. 279 
from the surface of 890 feet ;* the position of this boring is evidently close to the 
original margin of the Trias. The next (3), was at Ware, in which the Wenlock 
beds were entered beneath the Gault, so that the position of this boring is con- 
siderably south of the original margin of the Trias ; on the other hand (4), at 
Burford, the Triassic strata were passed through before the Coal-measures were 
reached, and they are also inferred by Professor Prestwich to exist under Oxford, as 
the water from St. Clement’s well is highly impregnated with chloride of sodium. 
From these data the concealed line of the old ridge can be approximately drawn, 
while the relations of the Secondary strata along the margin of the Mendip 
Hills and Somersetshire coalfield, enable us to determine its position there with 
-certainty.t 
Distribution of Land and Water.—The land of the period appears to have lain 
to the north-west, north-east, and south of the British Isles. The Highlands of 
England, Scotland and Ireland, were certainly in the position of land, and contri- 
buted to the sediment poured into the lacustrine area. In the south the sub- 
merged area was connected with that of Normandy and Brittany; but land probably 
lay over the region of Central and Western France. On the whole, the Triassic 
period over the region now described, was one in which elevation of land was at its 
highest at the beginning, and at its lowest towards the close, when the waters of 
the ocean invaded the tracts covered previously by those of large lakes.+ 
PuatE XXXII. 
The Jurassic Period. (Including the Rhetic, Liassic, and Oolitic Divisions.) 
At the close of the Triassic period the waters of the ocean invaded the tracts 
previously covered by lakes and estuaries. The influx of these waters is 
indicated by the fauna of the Rheetic (or Penarth) beds, which is marine, but 
indicates littoral and shallow water conditions. Along with Avicula contorta,§ 
Modiola minnima, Pecten Valoniensis, and Carduim Rheticum, there are remains 
of insects, fishes, and saurians; but none of cephalopods, whose habits require an 
open sea, With the commencement of the Lias, however, there occurred a general 
subsidence of the British and adjoining European area, upon which the sea 
established its supremacy over all but the elevated mountainous tracts ranging 
from Scandinavia into Britain. The waters brought with them great shoals of 
* Supra cit. 231. The late Mr. Samuel Sharp has also described this remarkable boring. 
+ This concealed ridge has been well described by Mr. Taylor in “ By-paths of Nature.” 
{ Ramsay, “ Phys. Geol. and Geog. of England,” 5th edi., p. 155. 
§ First described by General Portlock, in his “ Geology of Londonderry, &e.,” and afterwards identified 
by Dr. Wright, in Gloucestershire. Quart. Journ. Geol. Soc., vol. xvi. ; and Mr. Bristow, at Aust-Oliff 
and Penarth, on the banks of the Severn. 
