These Triassic rocks extend for about three miles, from Sand, on the east, to a point beyond Udrigil House, on the west. They are continuous, except near Udrigil, where the Torridon sandstone that encloses them comes to the surface. They are reckoned to be about a thousand feet thick. No fossils have as yet been found in them, but their age has otherwise been satisfactorily determined.
These rocks are extraordinarily interesting. They are the most northerly examples of the Secondary Geological Period on the west coast, and they form an isolated fragment of the deposits of this period, which once extended from Gruinard to the Ross of Mull to a depth of over a thousand feet, and which have been entirely swept away by enormous denuding forces, except at a few scattered points. Their protection has, in all cases, except at Gruinard, been due to being covered by volcanic outbursts on the grandest scale, which took place in the late Tertiary Period, and mainly formed the beautiful islands of Skye and Mull. At Gruinard, they were preserved from destruction by enormous faulting, by which they were dropped down at least a thousand feet into the Torridon Red. They are represented on the east coast of Sutherland, and, according to Professor Judd, by the famous reptiliferous sandstones of Elgin.[10]
Chapter X.
Minerals of Gairloch.
By Professor W. Ivison Macadam, F.C.S., F.I.C., M.M.S., &c., Edinburgh.
The following minerals were obtained in the localities mentioned, but the list is very incomplete. Time has not permitted of analyses being made of many samples, but such are now under examination, and will be available for a further edition of this work:—
Agalmatolite—Black Rock, Tollie.
Albite feldspar—Loch Fionn.
Agaric limestone—Coppachy.
Agate—Tollie Rock.
Barytes—Black Rock, Tollie.
Biotite—Loch Fionn.
Calcite—Black Rock, Tollie, &c.
Chalcedony—Glen Logan, &c.
Chalcopyrite—Coppachy.
Chlorite—Loch Gruinard, &c.
Dolomite—Glen Logan, Slioch, &c.
Epidote—Loch Fionn, Tollie Rock.
Galenite—Glen Logan limestone.
Garnet—Loch Fionn.
Heliotrope, or Bloodstone—Glen Logan, &c.
Hornblende—Loch Gruinard, Loch Fionn, &c.
Limestone (Massive)—Glen Logan, &c.
Limonite (Bog iron ore)—South Erradale, &c. &c.
Marcasite—Glen Logan, Coppachy, &c.
Muscovite—Loch Fionn, &c.
Oligoclase feldspar—Loch Fionn, &c. &c.
Pyrite—Glen Logan, Coppachy, &c.
Pyrolusite (Dendritic markings)—Loch Fionn.
Quartz—Common.
Quartzite (common)—Glen Logan, &c.
Rock crystal—Black Rock, Tollie (small crystals).
Serpentine—Black Rock, Tollie.
Smoke quartz—Tollie Rock.