252 SILTJKIA. [Chap. XI. 
Among the stratigraphical comparisons which have been made, Professor 
Harkness has shown that between the Bridge of Allan and Callender there 
is an elevated trough-shaped mass of the Old Eed Sandstone, in the lower 
portion of which conglomerates, followed by beds containing Cephalaspis 
and Pterichthys, are surmounted by red shale, grey sandstone, &c* 
Thus we see that in all the southern and central tracts of Scotland the 
lower beds are characterized by the same fossils ; and it will presently be 
shown that these beds have a true equivalent in the northernmost counties, 
but are there surmounted by a rich ichthyolitic group, the Caithness Flags, 
which is ill represented in the central or southern counties. 
Again, the classical work of Agassiz on the Possil Pishes of the Old Eed 
Sandstone has of late years received valuable additions through the labours 
of Sir P. Egerton, who has rectified the nomenclature by suppressing 
double names applied to the same Pish, and has bronght out in relief the 
true typical forms as described by Agassiz, Hugh Miller, M'Coy, himself, 
and others f. 
Northern Region of the Scottish Old Red Sandstone. — It has already been j 
shown that in one part of the South of Scotland the uppermost Silurian I 
ro.ck, with its fossils, is overlain conformably by red shale and sand- I 
stone (p. 161) %. Such an Upper Silurian basis is, however, wanting in the ' 
Northern Highlands ; and the reason is obvious. We now know that rocks 
of Lower Silurian age, associated with, and reposing on, old Cambrian con- 
glomerates, have there been metamorphosed into a crystalline state, and 
that a great change of the crust took place in the former outline of land 
and sea, in consequence of which, as we have every reason to think, no 
Upper Silurian sediment was ever accumulated. 
To determine, however, with precision the extent to which those crys- 
talline Highland rocks, whether quartzose, gneissose, micaceous, or 
chloritic, with subordinate limestones, may represent various Silurian 
subformations must still be a work of long labour. In the meantime it 
is clear that, ranging south-west from Sutherlandshire into Eoss-shire, 
they constitute in Loch Alsh and Kintail that lofty and steep watershed so 
near to the west coast of the Highlands, the strata of which, dipping 
rapidly to the E.S.E., are, after numerous undulations, transgressively 
surmounted by the lower member of the Old Eed Sandstone of the eastern 
counties (see the section at p. 169). 
"Wherever these two great systems of metamorphosed Lower Silurian 
rocks and the overlying Old Eed Sandstone are in contact, the direction 
of the one is completely discordant with that of the other. Thus, whether 
Director of the Montrose Society are extracts bottom-beds, and probably represents a portion 
from a description of the Basin of Montrose, by of the Caithness Flags. 
the Kev. Hugh Mitchell, with figures of the pe- * Quart. Journ. Geol. Soc. vol. xviii. p. 253. 
culiar Fishes of a band of the Old Ked Sandstone t See Quart. Journ. G-eol. Soc. vol. xvi. p. 119 
of Forfarshire, viz. Parexus incurvus, Agass., &c. 
Euthacanthus MacNicoli, Powrie, Climatius un- J See also Quart. Journ. Geol. Soc. vol. vii. 
cinatus, Egerton (Quart. J ourn. Geol. Soc. vol. xx. p. 168 ; vol. viii. p. 386. 
p. 422 &c). This band is above the Arbroath or 
