164 
SILTJRIA. 
[Chap. VIII. 
the geological structure of Cantyre, and stating that the so-called mica- 
slate of that tract is only a partially altered micaceous sandstone, which 
seems almost to pass upwards into the Old Eed Sandstone, he shows how 
all the other crystalline strata conform to and bend round the granitic nu- 
cleus of Goatfell in Arran. In confirmation of the probable identity of 
some of the crystalline strata of the north with the Silurian rocks of the 
south, he mentions the illustrative fact that calcareous matter prevails 
most extensively towards the western extremity of both groups of rocks. 
In the South-Scottish Silurian strata no calcareous beds are known at 
their eastern termination ; limestone only begins to appear in feeble 
courses in Peebles-shire, near the centre of the chain, and becomes very 
abundant (as above described) in Ayrshire. It is the same with the crys- 
talline strata of the north. In the coast-section from Stonehaven to 
Aberdeen, no calcareous bands occur ; in Forfarshire and Perthshire 
several are known ; they become still more abundant in the north of 
Argyllshire ; and in Cantyre (as in Ayr) we have a great group of lime- 
stone rocks. 
These views of the metamorphosis of the Lower Palaeozoic deposits into 
those stratified crystalline rocks which occupy so large a portion of the 
Highlands of Scotland have, since the publication of the first edition of 
this work, been reduced to a certainty as respects the north-western part 
of Sutherlandshire, by a discovery made by Mr. C. Peach. That keen-eyed 
collector, though residing far off at "Wick, had, in a rapid excursion to Dur- 
ness, detected there certain fossil shells in the crystalline limestone, which 
in that region is subordinate to quartz -rock. Now, according to Sedgwick, 
myself, and other geologists, including M'Culloch *, these quartz-rocks and 
limestones, associated with and passing under micaceous and gneissose 
schists, and resting upon ancient gneiss, are overlain on the east coast 
by the whole series of the Old Eed Sandstone, which is made up of the 
materials of such preexisting crystalline rocks : these will be treated of 
hereafter. 
Judging, then, from their relative geological position, of which, after an 
interval of twenty-eight years, I reassured myself by a visit during the 
summer of 1855, I had little hesitation in suggesting that these rocks 
were of Lower Silurian agef . In them my companion Professor Mcol 
and myself detected an Orthoceratite, but too imperfect to be referred to 
any known species. The Whorled Shell first discovered by Mr. Peach, 
many polished sections of which were examined, threw indeed a little 
more light upon the subject ; and specimens which the same zealous and 
unrivalled collector subsequently detected in these rocks, having been exa- 
mined and described by Mr. Salter, established the correctness of my view. 
The large Whorled Shell, one of which is here drawn, has proved beyond 
* See Sedgwick and Murchison, Trans. Geol. Soc. n. s. vol. iii. p. 125. 
t See Eeport of British Association, 1855, Glasgow, Trans, of Sections, p. 85. 
