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The Pamphlet Collection of Sir Robert Stout: Volume 22

The Metamorphic Series

The Metamorphic Series.

On the Mainland these may be grouped in two divisions, which are clearly marked off from each other by distinct lithological characters.

(a)Dark blue, green, and grey schists and clay-slates, with bands of quartzite and limestones.page 781
(b)Coarse-grained micaceous and hornblendic gneiss, with associated limestones, bands of quartzitc, talcose and micaceous schists.

These subdivisions are peculiarly serviceable to the glacialist, as they help him to determine the different movements of the ice during successive phases of the ice age. The representatives of the former series extend from Fitful Head northwards by the Bonxie and Cliff Hills to Laxfirth Voe; while the members of the gneissose series lie to the north-west of the area just described. They occur in the districts of Tingwall, Weesdale, Nesting, Lunnasting, Delting, and along the eastern seaboard of Northmavine. The strike of these motamorphic rocks is generally N. 10°–20° E.; and though opposing dips are frequently met with, indicating repetitions of the strata, they usually dip to the north of west at high angles. Hence we have a gradually ascending series from the schists and clay-slates of the Cliff Hills to the coarse micaceous gneiss west of the vale of Tingwall, and the massive limestones of Whiteness and Weesdale.

To the persistent trend of the metamorphic rocks must be ascribed the remarkable ridge-shaped contour of the ground in the centre of the Mainland. The coincidence between the trend of the strata and that of the parallel ridges seems to indicate a direct relationship between the two, the denuding agents being guided in their operations by the relative hardness and softness of the materials exposed to their influence. Hence it follows that we have a scries of intervening hollows running parallel with the ridges, which usually terminate seawards in long narrow voes or sea-lochs. The erosion of these hollows has doubtless, in some instances, been duo to the partial removal of the bands of limestone by the chemical action of carbonated waters, inasmuch as the outcrop of the limestones coincides with the course of a longitudinal hollow.

The coarse-grained gneiss of Whalsey and the Outskerries, with the associated limestones, is merely the prolongation of the Mainland series; and the same remark is applicable to the gneiss occuring in Yell.

The structure of Unst and Fetlar is somewhat different, inasmuch as these isles contain well-marked zones of serpentine and gabbro, the distribution of which has an important bearing on the question of the dispersal of the stones in the Boulder-clay. In the island of Unst, the Vallafield ridge which flanks the western seaboard, whose highest elevation is about 697 feet, is mainly occupied by coarsegrained gneiss, dipping to the south of cast at comparatively high angles. On the eastern slopes of the ridge the gneiss is succeeded by grey mica-schists and green chloritic schists, and these are overlain in turn by black graphitic schists. These dark schists seem to form a reliable horizon with reference to the masses of serpentine and gabbro, as they usually crop out along the margin of the areas oeeupied by these masses and generally dip underneath them. Though these schistose rocks form but a narrow band from Belmont Bay northwards to Baliasta, they occupy a much broader area to the north of the latter point, constituting, in fact, the group of hills page 781 round Saxavord. They reappear again in the south-east corner of the island, where they cover a strip of ground about a mile in breadth between Skuda Sound and the ruins of Muness Castle.

The masses of serpentine and gabbro in Unst lie in a trough formed by these schists. They may be said to form two parallel zones crossing the island from south-west to north-east, the serpentine lying to the west of the gabbro. The serpentine area is the larger of the two, though somewhat irregular in outline; at the northern limit between Baliasta Kirk and Swena Ness, the mass is nearly two miles in breadth, but as it is traced southwards it diminishes to half a mile in breadth. Another patch of gabbro is to be met with on the promontory east of the ruin of Muness Castle.

It seemed to us that the serpentine has resulted from the metamorphism of the gabbro. Hero and there in the gabbro area, as, for instance, on the west side of Uya Sound, lenticular patches of serpentine occur, as if the transmutation had partly begun and had been interrupted. The gradual transition from the one rock to the other is well seen in the promontory on the south side of Balta Sound. Professor Heddle, who advocates this view, states that the gradual passage can be seen in hand specimens on Swena Ness.

The structure of the northern portion of Fetlar is comparatively simple. The central hollow coincides with a low anticlinal axis of black graphitic schists and chloritic schists similar to those in Unst, and apparently occupying the same horizon with reference to the gabbro and serpentine. These rocks throw off on both sides of the arch beds of gabbro and serpentine, forming the elevated ground round the Yord Hill on the east and the hills near Urie on the west. At Urie the serpentine which overlies the gabbro is immediately succeeded to the west by coarse-grained gneiss, the perfectly conformable junction between the two being distinctly visible on the shore west of the promontory of Urie. The broad mass of serpentine which stretches from the Vord Hill eastwards to Grating Bay is thrown into a synclinal trough, which is nowhere deep enough to bring in the overlying gneiss to the west of Urie. To the east of Gruting Bay occur the micaceous and chloritic schists as well as the graphitic schists, which contain in minor folds small patches of serpentine.