OLD RED SANDSTONE OF WESTERN EUROPE. 357 
to believe the present southern margin of the Old Red Sandstone area to mark 
the coast-line, not of the earlier but of the later part of the history of Lake 
Orcadie ; in other words, that for a long time the area of the Moray Firth was 
land, and that only towards the close of the existence of that lake, by a gradual 
submergence of the area, did the water creep southward, and accumulate the 
present marginal deposits. 
2. WoRK OF PREVIOUS OBSERVERS. 
Before entering upon the history of this great northern basin of Old Red 
Sandstone, which in many respects, and notably in regard to its organic remains, 
is one of the most interesting areas of that geological system which has yet 
been noticed, let me refer briefly to the labours of previous observers, and to 
the present state of our knowledge on this part of my subject. 
While the general area covered by the Old Red Sandstones and con- 
glomerates in the north of Scotland had already been so well traced as to find 
tolerably accurate expression in the geological map, published by Bours, as far 
back as 1820; there does not appear to have been any attempt to work out. the 
structure and subdivisions of the system in that region until the year 1827, 
when Sepewick and Murcuison examined the wide belt of country extending 
from the west of Ross-shire, northward through Sutherland, eastward across 
Caithness, and then southward by the shores of the Dornoch, Cromarty, Beauly, 
and Moray Firths.* In the important Memoir which gave the results of their 
labours, these authors not only traced more accurately the area covered by the 
Old Red Sandstone, but ascertained the general order of succession of its com- 
ponent groups of strata, noted some of their fossil contents, and showed the 
probable connection of the fossiliferous deposits even at the opposite extremi- 
ties of the district. As the larger part of their Memoir deals more particularly 
with the formations as these are exhibited in Caithness, this portion of it will 
be again more specially referred to when the Caithness development of the 
system is treated of. Sepewick and Murcuison considered the Old Red 
Sandstone of the north of Scotland to consist essentially of three groups,—1s¢, 
A mass of red conglomerate and sandstone forming the base, and lying 
unconformably upon the primary rocks, from the waste of which it had been 
formed ; 2d, A middle series consisting, in Caithness, of a thick pile of calcareo- 
| bituminous schists with fossil fishes, and along the Moray Firth of red and 
| grey sandstone, marls, and calcareous nodules, and cornstone; 3d, An upper 
| set of light yellow and reddish sandstones. They remark that while the con 
* “Trans, Geol. Soc.” 2d ser. vol. ii. p. 125. 
VOL. XXVIII. PART II. 5A 
