168 
SILUEIA. 
[Chap. VIII. 
feature of the reform in the classification of the older rocks of the North of 
Scotland stands out among the most prominent geological advances which 
of late years have been made in Britain ; and I rejoice that the hypothetical 
views I had for some time entertained were borne out by a rigorous appeal 
to nature, as now attested by my associates Professors Eamsay and 
Harkness and Mr. Geikie. 
As the age of the limestones and quartz -rocks of the North Highlands, 
with their associated mica-schists &c, has been a qucestio veocata — the 
lamented Hugh Miller having hypothetically suggested that they might be 
the representatives of the Old Red Sandstone and Caithness flags of the 
east coast, and Professor Nicol having at one time considered them to be 
metamorphosed Carboniferous rocks* (neither of which views can be sus- 
tained) — it is satisfactory to me to find that the old view of Professor 
Sedgwick and myself, with respect to their relative physical position, has 
been supported by other observers and the independent evidence of fossils. 
This discovery of North- American Lower Silurian fossils in the northern 
extremity of Scotland is also of deep importance in general geological rea- 
soning ; for it will be hereafter shown that many of the same species of 
organic remains in rocks of this age extend throughout certain latitudes, 
over areas very remote from each other — or, in other words, that the 
Scandinavian, British, and North- American f Silurian rocks have the same 
typical characters. 
When the first edition of this work was published, I entertained the 
belief, in common with my early associate Sedgwick and our precursor 
M'Culloch, that the striking mountain-masses of red conglomerate and 
hard grit on the north-western coast of the Highlands %, which rest in 
layers more or less horizontal on low and gnarled bosses of highly inclined 
and ancient granitoid gneiss, as represented in the vignette at p. 170, were 
really a part of the Old Red Sandstone of Scotland §. Re-examination, 
however, of those tracts ||, compelled me to abandon that view. I then 
saw that these conglomerates, resting on the oldest or granitoid gneiss of 
the Highlands, as around Loch Assynt, were really inferior to those quartz- 
rocks and limestones in the prolongation of which to the north coast of 
* Quart. Journ. G-eol. Soc. vol. xiii. p. 36. _ merate and grit as underlying to some extent the 
t The great Maclurea of the Lower Silurian quartz-rocks and limestones of Sutherland. At 
rocks of Ayrshire has been identified by Professor that time no one had begun to classify any fos- 
M'Coy with the M. magna, Hall ; and in Ireland, siliferous strata below the Old Eed Sandstone, 
one of the Lower Silurian Trilobites is an Ameri- and all the old Scottish conglomerates, which are 
can species, the Isotelus gigas, Dekay. now being referred to Cambrian, Silurian, or De- 
X My first visit to the west coast of the High- vonian ages respectively, were then grouped with 
lands was made in 1826, accompanied by Lady the Old feed Sandstone. Having been prevented 
Murchison, during which my attention was almost from detecting the unconformity on the west coast 
exclusively devoted to descriptions of the Oolitic which has since been noticed, we naturally con- 
and Liassic strata of the E. and W. of the High- eluded, with M'Culloch and others, that the con- 
lands (see Trans. Geol. Soc. 2nd ser. vol. ii. p. 293 glomerate of the west coast was of the same age 
&c). In the following year Professor Sedgwick as that on the east,— an error which will be 
and myself made that survey which enabled us to specially dwelt upon in the 11th Chapter, when 
write the memoir upon the Old Eed Sandstone of treating of the Old Eed Sandstone. 
Scotland, which is printed in the Geological Trans- § See M'Culloch's Western Islands of Scotland; 
actions (n. s., vol. iii. p. 153). Although we en- Cunningham's Mem. Highland Society, vol. xiii! 
countered bad weather in Assynt, which prevented p. 73 ; N icol, Journ. Geol. Soc. vol. xiii. p. 17. 
us from making accurate distinctions among the || Accompanied by Professor J. Nicol, in 1855. 
rocks of that tract, we even then noted a conglo- 
