Palao-Geological and Geographical Maps of the British Islands, 259 
the margin without much change in their characters ; of these two phenomena the 
Carboniferous and Jurassic limestones offer striking examples.* 
In determining the position of the land surfaces, and submerged areas during 
successive geological periods, one of the most important guides is the discordant, or 
unconformable, relations of the strata to those of older date. Where two sets of 
strata, such as those of the Triassic and Silurian periods, are highly discordant to 
one another, there is much probability that the older formation was in some region in 
the position of a land surface when the newer formation was being deposited under 
the waters of an adjoining sea or lake. In this case the actual margin is frequently 
indicated by the abrupt uprising of the older formation with reference to the newer, 
allowance being in all cases made for the effects of denudation. Ifthe two un- 
conformable sets of strata immediately succeed each other, or are in close geological 
sequence, then the marginal relations are more easy of determination. Such is the 
case, for instance, with the Lower and Upper Silurian strata, which, though highly 
discordant as regards stratification, are in immediate geological sequence, and the 
original marginal beds, those of the upper Llandovery age, are found in the form of 
conglomerates not far removed from their position as shore beds.+ ‘Throughout the 
whole succession of formations from the Cambrian to the Cretaceous there is no 
more marked physical break than that which occurs between the Lower and Upper 
Silurian formations over the area of the British Islands. 
The occurrence of beds of conglomerate, or breccia, is generally an indication of 
littoral conditions, and indicates the proximity of the land of the period to which 
they belong. Thus, the breccias which occur in the New Red Sandstone in two 
stratigraphical positions in Shropshire and Worcestershire, and which thin away 
eastward, indicate the proximity of the marginal land formed of Devonian, Silu- 
rian, and Cambrian rocks, in the adjoining districts west of the Severn. The 
conglomerates of the Old Red Sandstone along the southern slopes of the Gram- 
pians, and in other places, point to similar conditions; but it is remarkable to how 
great a distance rounded pebbles have been transported in some instances from 
their original sources ; as, for example, in the case of the New Red Conglomerate 
of the central counties of England, the source of supply for which was apparently 
in Scotland. 
When engaged in the attempt to restore the physical features of successive 
geological periods over the region embraced in this treatise, [ became forcibly im- 
pressed with two leading ideas. First, that the present North Atlantic Ocean, 
* Some years ago I illustrated these phenomena in a paper on “ Isodiametric lines, and the relative 
distribution of the calcareous and sedimentary beds of the Carboniferous system.’—Quart. Jour. Geol. . 
Soc., vol. xviil., p. 127. 
+ This can be observed both in Wales and the West of Ireland. 
+ On the origin of the quartzite pebbles of the Bunter Conglomerate, see “‘ The Triassic and Permian. 
Rocks of the Midland Counties of England,” p. 59.—Mem. Geol. Survey (1869). 
DIE G 
