Chap. XI.] 
BASE OF THE OLD EED SANDSTONE. 
261 
marty and Caithness. On looking, however, to the physical order of the 
masses in that northern region (section, p. 255), we see that this view can- 
not he retained ; for the "bituminous schists of Caithness are comparatively 
high in the series, and, resting upon a great thickness of sandstone and 
conglomerate (a, b, c), are overlain hy the upper zone of the group only. 
According to my view, therefore, as founded upon a clear order of infrapo- 
sition, and also on the occurrence of a Pteraspis, the conglomerates and 
sandstones which underlie the flagstones of Caithness {a, b, of section, 
p. 255) are clearly the equivalents in time of the lower cornstone strata of 
England. This rectification is of considerable importance, since good geo- 
logical text-hooks, until very recently, following Hugh Miller, have placed 
the Caithness Mags in the lowest division of the Old Red Sandstone*. 
We may also, indeed, clearly infer that although the Arbroath paving- 
stones, with their Pterygoti, do not represent the uppermost Ludlow rocks, 
still it follows that the Cephalaspis-beds of Forfarshire must fall into the 
lower division of the Old Red group. 
In Shropshire and Herefordshire the true base of the Old Red Sand- 
stone, properly so called, is a red rock containing Cephalaspis and Pte- 
raspis (see pp. 141, 244 et seq.) and gradually passing down into the grey 
Ludlow rock ; and in both of these contiguous and united strata, remains 
of large Pterygoti are found, but of different species in the two bands. 
Now, although the Arbroath paving-stone and the grey rocks ranging to 
the north of Dundee much resemble lithologically the uppermost Ludlow 
rock, they contain the Cephalaspis Lyellii, with another kindred species, 
and are therefore to be classed with the Devonian rocks, though they 
must, under all circumstances, be viewed as at or near the base of that 
natural group f . In speaking of the oldest member of the Old Red Sand- 
stone as characterized by the Cephalaspis Lyellii, I repeat my conviction 
that, in the North-eastern Highlands and Caithness, this lower zone is 
represented by the vast thickness of thin-bedded red sandstone and con- 
glomerates with Pteraspis which has already been adverted to as lying 
beneath the Caithness flags. (See Quart. Journ. Geol. Soc. vol. xiv. 
p. 503.) 
In the autumn of 1859 I examined the eastern flank of the Grampians 
with Mr. Powrie, having previously surveyed the coast of Forfar ; I then 
satisfied myself, by finding the Parka decipiens in schists subordinate to 
* In the last or 6th edition of the Elements of shire zone, which I consider to be higher in the 
Oeology, p. 527, Sir Charles Lyell has admitted series, and the true base of the Old Eed or De- 
the validity of my view. vonian, has been enriched, through the labours 
t In a communication made to the British As- of Mr. Page and Mr. Powrie, by the addition of 
sociation at Leeds, in 1859, Mr. D. Page offered two new Crustaceans, Stylonurus and Campecaris, 
some " Further Contributions to the Palaeontology and by the discovery of parts of the Ichthyolite 
of the Tilestones, or Silurio-Devonian strata of Cephalaspis Lyellii, and of another fine species, 
Scotland." He confirmed my original view (see as well as many fish-spines &c. The term ' Tile- 
pp. 160 et seq.) that the Lanarkshire black schists stones/ for which I have substituted 'Passage- 
are Upper Silurian, and added many fossils, col- beds,' cannot, therefore, be made to include both 
lected by Mr. Slimon, viz. Pterinea, Orthonota, the black schists of Lanarkshire (which are un- 
Nucula, Avicula, Spirorbis, &c, together with new questionably Upper Silurian) and the Arbroath 
forms of Stylonurus, a Crustacean allied to Eu- paving-stones (which must be regarded as the 
rypterus, all of which sustain the inference I for- basement-beds of the Old Eed). 
merly arrived at. The paving-3tone or Forfar- 
