Its capabilities in this way are also well exhibited on the west coast round Loch Assynt, rising there into the summits of Beinn More and Queenaig, above three thousand feet; and also near Loch Carron to the south, and between Assynt and Eriboll to the north. On Loch Torridon, its prevailing tendency to whiteness gives rise to the name Grey Heads, very descriptive of certain contorted peaks near Coulin Lodge.
The Quartzite is interesting as exhibiting the earliest indications of organic life yet discovered in Scotland:—
1. Annelid Borings.—The lower beds next the Torridon contain, on their surface, as described by Murchison, "large round knobs on the top of cylindrical bodies, which pass through several layers," their number being often astonishing. These are, it is safely concluded, "infillings of excavations" made by certain worms called Annelids, and are known as Annelid Borings. They are noteworthy as "the oldest vestiges of life which can be detected in the North Highlands." They are often very clearly seen, as the filling in has generally been done by a different coloured sand from that in which they had been bored. They sometimes project above the surface like "pipes," and are so numerous as to cause these beds to be called "pipe-rock." Examples are abundant round Kenlochewe, and on the roadside at the entrance to Glen Cruaidh Choillie, where they are unusually good. They should be secured by the intelligent visitor from their extraordinary interest.
2. Fucoid Remains.—Interstratified with the Quartzite, are certain brown, mottled, shaly and flaggy bands, with curious impressions of what seem leaves, which have been thought to be fucoids or seaweeds. The recent Survey explorations would seem to point to their being simply very much squeezed annelid "pipes." The shales in which they occur are thus generally known as the Fucoid Beds, and, when found, are very good evidence of the horizon of the rocks. They are often very distinct and easily seen, and are most interesting. They occur on Loch Maree near the top of the east side of Glen Fasagh, imbedded in the Quartzite, and run through the Quartzite to Glen Cruaidh Choillie.
Other organisms have been found in it elsewhere, such as orthoceratite in Assynt, and certain small conical bodies called serpulites.
This Quartzite, with its annelid borings and fucoid beds, is placed by Murchison in the Silurian series, the third in the geological record. By others, such as Dr Hicks, it is considered possibly Cambrian.
IV. The Limestone.—On the western side of Glen Cruaidh Choillie, resting on the Quartzite, and generally conformable with it, is found a limestone. By examining the [map], it will be seen that this limestone runs more or less continuously from Loch Carron to Loch Eriboll. It receives its greatest development at Inchnadamph, at the east end of Loch Assynt, where it forms splendid cliffs. It is of commercial value, and has been worked at various places along its outcrop. It will also be observed that there is a wide isolated patch of limestone at Durness, between Loch Eriboll and Cape Wrath.
In this Durness limestone, which was long considered unfossiliferous, like the other rocks of the North-West Highlands, shells were discovered in 1854 by Mr Peach, the eminent geologist, and friend of Dick of Thurso. These were determined to be Silurian by Mr Salter, a great specialist in such matters, and were described and figured in a paper by Sir Roderick Murchison in 1858.[5] Since then finer specimens have been discovered. Their likeness to British Silurian fossils is very remote, and they are more related to American forms; but they are generally now accepted as of Silurian or Ordovician age. This discovery of fossils gave a great impetus to the study of these rocks, and formed the basis of the theory propounded by Sir Roderick Murchison.
The Durness limestone turns out, however, to be, as a whole, of a different type from the great strike of limestone which goes through Glen Cruaidh Choillie and terminates at Loch Eriboll. This Durness limestone is held by Dr Heddle, who first ascertained the fact, and by other competent authorities, to be non-dolomitic, while that of the great strike to the east is dolomitic; dolomite (so called from the French geologist Dolomieu) being a variety of limestone, which, in addition to the carbonate of lime of which common limestone mainly consists, contains more or less carbonate of magnesia,—in this dolomite, forty-eight per cent. Dolomitic beds have, however, lately been discovered in the Durness basin by the Survey. For long, no fossils were obtained from the great dolomitic strike, except an orthoceratite at Assynt by Mr Peach, and a possible organic mass by myself at the same place; but recently a varied and important suite of fossils has been gathered by the Survey, which has clearly decided the age of the Dolomite to be Silurian. Of its position above the Quartzite there is no doubt.
It is pretty well exhibited in Glen Cruaidh Choillie, where it has been worked at various places, and where its superposed junction with the Quartzite can be seen.