On an apparently new Mineral occurring in the Rocks of Invernesshire

Abstract

abstract I. Its Geological and Geographical Distribution The Moray Firth, from Duncansbay Head to Buckie, is enclosed by the Old Red Sandstone, except some Jurassic patches near Brora and Cromarty, and others near Elgin. In this area it is touched by no Silurian except for a few miles between Inverness and Beauly. The Old Red Conglomerate and other beds exist west of Inverness in a triangular patch between Dochgarroch and Bunchrew. The Silurian to the west of this forms an oblong semidetached area surrounded by Old Red, except for a few miles on the south. This Silurian patch has a general slope to the Moray Firth on the north, into which it is drained. Its main stream is the Moniack Burn, rising three miles north of Drumnadrochit in Glenurquhart, running generally along its centre and falling into the sea west of Lentran. The strata form a syncline, the axis striking roughly N. and S., the rocks on the east dipping to N.W., those on the west dipping generally to S.E., but the latter much disturbed. They consist chiefly of clay-slate, micaceous and chloritic schist, and gneiss. A limestone, worked at several points, runs parallel to and near the axis, of varying thickness, sometimes twelve feet, and often concretionary and interbedded with other rocks. A red granite also occurs in the Silurian near Abriachan; it is of commercial value, and known as Loch-Ness granite. Within this Silurian area the mineral which was the subject of the paper occurs at five points, at three of these in situ , and and has as yet been found nowhere else. I t is of a blue colour of various shades, from ultramarine to bluish white, often striking and beautiful. It occurs generally in a felspathic matrix, readily disintegrates under water to a fine, blue, soapy, unctuous clay (caused probably by the magnesia it contains), and is seldom found pure, and never yet in a crystalline form, though it sometimes presents a glistening crystal-line aspect. The sites where it has been found are these :— (1) Englishton Moor .—On the west side of Bunchrew Burn, a little above the Public School of Kirkton, at No. 23 croft, near an outcrop of limestone. It occurs here only in scattered blocks evidently carried from the west at Moniack Burn, where it is in situ . It is chiefly found in thin veins and plates in felspar. It was here that the mineral first attracted special attention, during an excursion of the Scientific Society and Field Club of Inverness, on September 1st, 1877, at which both the authors were present—Mr. Jolly having previously noted it in a preliminary examination of the ground along with Mr. Cran, of Kirkton, and Mr. Cameron having, at the excursion first suspected its rarity and subsequently analyzed it. (2) Moniack Burn .—This passes through a very picturesque gorge called Reelig Glen, enclosed by high cliffs, finely wooded, and attractive to the geologist and botanist. The stream runs here along a fault, near the synclinal axis, associated with a remarkable conglomerate and with granitic, felspathic veins. A great fall of rock took place last winter from the face of a high precipice in the glen above Reelig House. The fallen debris contained much of the mineral, associated with orthoclastic felspathic rock, interbedded with clay-slate, and mica and chloritic schist, &c., not only in veins but in regular strata some feet in thickness. (3) Near South Clunes Farm .—It is also found in situ above this gorge, on the east bank of the stream, at a limestone-quarry, where it occurs in great purity in a felspathic rock in contact and bedded with the limestone. It seems here to be more or less associated with the limestone, which crosses the river from this point near the fallen cliff, and runs to Rebeg farm, where also it is worked. (4) Near Dochfour, at the north end of Loch Ness —It occurs here in rock in situ , on a new road to the Mansion House, about 150 yards north of a new bridge over Dochfour Burn. It is also associated hero with felspar in large masses, bright blue at first but gradually losing colour by exposure, and easily disintegrating under water. (5) At Lochend, at the north end of Loch Ness .—It was found here, not in situ , but in detached blocks in the burn near Lochend Hotel, by Dr. Aitken and Mr. Wallace, members of the Inverness Field Club. The blocks have probably come down the burn, but their source has not yet been discovered.