266 Paleo-Geological and Geographical Maps of the British Islands. 
presented south of the Bristol Channel, by the “ Foreland grits and slates” of 
North Devon.* 
These beds were formed by accumulations, sometimes of great thickness, of 
green, red, and purple sandstones, grits, shales, or slates, and conglomerates—of 
marine origin, in the southern portion of the British area, but, in the northern, 
probably of lacustrine origin. In the latter district, according to the views of 
Professor A. Geikie, the beds of this division of the series were deposited in 
several distinct lake-basins. One (“L. Orcadie”), north of the Grampians ; a 
second (“L. Caledonia”), south of these mountains ; a third (“ L. of Lorne”), a 
district north of Argyleshire, lying at the entrance of the Great Glen ;+ and a 
fourth (“L. Cheviot”), on the southern borders of Scotland.{ These deposits 
were derived from the waste of the adjoining lands formed of the metamorphosed 
beds of the Highland mountains, but how far they extended in the direction of the 
Scandinavian promontory is altogether uncertain, so that the eastern limits of these 
basins must be left undefined. | 
Relations to the adjoining Formations —Throughout the British area the Upper 
Silurian beds are unconformable to the Lower Silurian, and in some cases, as in 
Shropshire, they rest directly on Cambrian beds. In a word, the physical hiatus 
between the upper and lower divisions of the great Silurian system of Murchison 
is as marked and complete as it is possible to conceive between any two adjoming 
sets of strata; and this being the case, it is not to be wondered at that Sedgwick 
claimed as “Cambrian” all the beds below the Llandovery horizon. After the 
close of the Lower Silurian epoch, represented by the “ Bala Beds,” there occurred, 
over the region in question, terrestrial disturbances of great intensity, accom- 
panied in the north of Ireland and Scotland by metamorphic action.g Large 
tracts of the ocean bed were converted into land surfaces, while denudation 
ensued on a great scale, owing to which the uppermost Lower Silurian beds were 
washed away, and on the resubmergence of the depressed tracts, these materials were 
used up in the construction of the basement beds of the succeeding Upper Silurian 
series. Depression then went on, during which the Wenlock and Ludlow beds were 
formed under tranquil waters, and towards the close of the latter period, the 
* “On a proposed Devono-Silurian Formation,” Quart. Journ., Geo. Soc., May, 1882, p. 200; also 
Trans. Roy. Dub. Soc., vol. i., antea p. 147 (1880). 
+ < Qn the Old Red Sandstone of Western Europe.” Part I., Trans. Roy. Soc. Edin., vol. xxviii. 
The margins of these basins drawn on the maps (Plate XXV., Fig. 2), are very much those indicated by 
Prof. Geikie. He considers that Lakes “‘ Orcadie ” and “ Caledonia” were never united. 
It seems probable that in its earlier condition this lake was connected with the sea, but was subse- 
quently disconnected. 
§ That this metamorphism of the Lower Silurian beds of the North British area took place before 
the Upper Silurian period was first pointed out by Harkness in his paper ‘‘ On the Age of the Rocks of 
West Galway,” &c. Quart. Journ. Geol. Soc., vol. xxii., and has more recently been insisted on by myself 
in the “ Phys. Geol. and Geog. of Ireland,” p. 22 (1878). 
