258 SILURIA. [Chap. XI. 
of Scotland, has been made by Mr. Charles Peach. That successful fossilist | 
detected in strata near Lybster, much older than the Caithness flags, j 
the remains of a portion of a Pteraspis, — an Ichthyolite never yet found ! 
above the lowest of the three divisions of the Old Eed Sandstone. 
The red sandstone of this lowest zone, assuming gradually a flag-like I 
character, and becoming of a purplish tint, passes up by other beds, c, into 
those grey- coloured flagstones which, forming a large portion of the county 
of Caithness, are so well known to geologists through the description of 
their imbedded fossil remains by Agassiz and Hugh Miller. From the 
environs of Lybster, where the strata (d of the diagram at p. 255) begin 
to contain fossil Fishes, these flagstones present a general uniformity 
in their lithological characters ; and, though here and there so calcareous 
as to have been in parts formerly burnt for lime, they are, on the whole, 
to be regarded as a great series of hard, argillo -siliceous beds of great 
tenacity, in which, within the last forty years, large and valuable quarries 
have been opened out. 
The most satisfactory study of these flagstones, from their lower members 
near Dunbeath to their central parts at Wick, and thence to their highest 
strata on the shores of the Pentland Frith, where they pass up into, and 
are covered by, the upper red sandstones of Dunnet Head, is to be made in the 
precipitous coast cliffs, which, abraded by the surges of the ocean, exhibit 
many remarkable small bays and headlands, on the sides of which the j 
hard flagstones present their rough and jagged edges*. The thickest 
beds, near Wick, are from 14 to 15 inches thick, and, when quarried for 1 
building-purposes, occasionally furnish blocks of 20 feet in length, which I 
are of great tenacity and durability. In some of the calcareous courses 
Mr. C. Peach has discovered that the fragments of fossil Fishes have in 
certain spots been ground down to a powder, which constitutes a large 
portion of the matrix of the rock. 
Many of the beds are so bituminous, owing to the quantity of animal 
matter they contain, that even when described by me in 1826 they were 
termed the ' bituminous schists ' of Caithness, bitumen being seen to exude 
naturally from themf. Having obtained specimens from near Barro- 
gill Castle, collected by Mr. Peach, I submitted them to analysis by Dr. 
Hofmann, who, according to the report given in the Appendix C, attaches 
considerable value to this mineral if it be obtainable in sufficient masses. 
Now, as it has been found profitable to distil ' stone-oil,' or petroleum J, 
from beds of Secondary age charged with fossil Fishes, in high recesses of 
the Alps, we may certainly infer that these Palaeozoic strata lying so near 
the sea may yield so much bitumen that, in consequence of it and the 
* Not having personally examined all the coast These features are not represented in the general 
between Wick and Latheron Wheel, I requested section, p. 255. 
Mr. C. Peach to do so; and he detected the ex- t See Trans. Geol. Soc. Lond., 2nd ser., vol. ii. 
istence of an axis at Sarclet, by which conglo- p. 314. 
xnerates are again brought up, and are thrown off J See my paper on the Bituminous Schists of 
to the South and to the North, thus passing on Seefeld, Proc. G-eol. Soc. Lond. vol. i. p. 39. 
cither side under the fish-bearing flagstones. 
