JAMES REID ON MARINE ORIGIN OF OLD RED SANDSTONE. 23 
In our view, however, as we shall subsequently show, the evidence 
points strongly to a “marine” origin. The Laurentian and Cambrian 
formations are supposed to have formed a northern continental land, 
abutting upon which the Silurian, and doubtless the Devonian, 
systems were consecutively laid down. In the British area the Silurian 
land apparently culminated in the Grampian range, whose southern 
base was laved by the waves of the Devonian sea. We know that 
the depth of the Old Red Sandstone of “ Lake Caledonia ” exceeds 
20,000 feet, which was admittedly formed in a sinking area. Now the 
theory which requires our belief in an inland sinking of the land to 
this extent, with the sea of the period shut out, seems in our view 
too strained and improbable for acceptance. 
In the Old Red Sandstone of the midland valley of Scotland 
we therefore appear to have a typical lake basin, and in the extracts 
above referred to a descriptive account of its origin is given. But 
if we are to infer that the Lammermuir range on the south side of 
the basin formed land during the greater part of the Old Red Sand¬ 
stone period, the evidence is unquestionably against it. That the 
Lammermuir range, on the contrary, was under water during the 
greater part of the Old Red Sandstone period is made evident by the 
following estimate, made by an undoubted authority, of the amount 
of throw of the great fault which stretches across Scotland from 
Ayrshire to Midlothian, and by the inference which that authority 
deduces therefrom :—“ From the detailed survey of the Old Red 
Sandstone it is possible to estimate the amount of throw the fault 
has here, . . . we know the thickness of that formation to be 
fully 15,000 feet, and the fault must be a dislocation to that amount 
at least, ... so vast a thickness of Old Red Sandstone could 
not have ended originally where the fault now is, but must have 
swept southwards over the lower Silurian uplands.” * 
In these 15,000 feet of deposits we have the equivalent in time of 
at least three-fourths of the Old Red Sandstone period, and in the 
deposition of this mass of strata the fact is implied that the Lammer- 
muirs during the same period were, not above, but under the sea. 
The southern uplands, unlike the central Grampian range, show 
evidences—if we may so express it—of an “unstable continental 
margin ” during Old Red Sandstone and Carboniferous times. Cor¬ 
roborative proof of this is afforded in “ the south-westward extension 
of the Silurian ridge of the south of Scotland into Ireland, where it 
sinks beneath the carboniferous rocks of Connaught.” f 
To the north of the Grampian range a similar occurrence is seen, 
where, in the mountain chain skirting the southern borders of Caith- 
* Explanation of Sheet 15, Geol. Survey of Scotland. 1871. Page 37. 
t Prof. Young, Introduction to Cat. IV. Scot. Fossils. 1876. 
