e Cephalaspis beds of
Forfarshire placed in its middle, and the Holoptychius beds of Scot-Crag
and Clashbinnie on its upper horizon; but since that time the tilestones
have been transferred to the Upper Silurian division of rocks, and the
evidence furnished by their supposed Dipterus has not been confirmed.
And as the Old Red Sandstones of Scotland have no true fossiliferous
base, but rest on primary rocks both to the south and north of the
Grampians, it may be regarded as in some degree a moot point whether the
lowest fossiliferous beds to the north be older or newer than those to
the south, or, what is quite possible, of the same age. Provisionally,
however, I have arranged my paper on the supposition that the Coccostean
formation of the north is the lowest and oldest of the three; and this
from the following considerations. In the first place, the Coccosteus
and its contemporaries appear in the north at a very short distance
above the base of the system. I have disinterred an Osteolepis from a
fish bed near Cromarty only thirty-three feet over the great
conglomerate, and only a hundred and twenty-nine feet over the granitic
gneiss beneath; whereas the Cephalaspis beds occur high above the
primary base of the system in the south,--at some distance over even the
thick conglomerate of Stonehaven and Dunnottar; and under this
conglomerate, as shown in the section furnished by the valley of the
North Esk, there lies a pale red sandstone member of the system,
estimated by Colonel Imrie at seven hundred and eighty feet in
thickness. The conglomerate itself he estimates at twelve hundred feet.
Adopting as correct Colonel Imrie's section, taken along the banks of
the North Esk,--and the colonel was unquestionably a truthful
observer,--the Cephalaspis beds of the south lie nearly two thousand
(nineteen hundred and eighty) feet above the Azoic slates on which the
Old Red Sandstone of Forfarshire rests, whereas the Coccosteus and
Osteolepis beds of the north lie only one hundred and twenty-nine feet
over the Azoic gneiss on which the Old Red Sandstone of Cromarty rests.
There is thus at least _room_ in the south for an underlying
fossiliferous formation between that of the Cephalaspis and the base of
the system, but none in the north beneath that of the Coccosteus and its
base. In the north we find the _room_ lying above, between the
Coccostean and Holoptychian formations, and represented by that great
unfossiliferous deposit of