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The Pamphlet Collection of Sir Robert Stout: Volume 22

Intrusive Igneous Rocks in the Metamorphic Series

Intrusive Igneous Rocks in the Metamorphic Series.

Under this heading we shall only indicate those intrusive rocks and those products of extreme metamorphism which are probably older than the Old-Red-Sandstone period. We have already referred to the areas of gabbro and serpentine in Unst and Fetlar; but in addition to these there are certain masses on the Mainland deserving special notice.

Of these by far the largest is the mass of diorite occurring in the districts of Delting and Northmavine on the Mainland. It is upwards of ten miles in length, and in places it exceeds two miles in breadth; but it ought to be borne in mind that the whole of the area now described is not occupied by the diorite, nor is the boun- page 783 clary line so uniform as we have represented. A minute examination of this tract convinced us that the groundwork of the area, so to speak, is formed of metamorphic schists, which are traversed in all directions by largo and small veins of this rock. Both the diorite and the schists are intersected by innumerable veins of quartz-felsite which were injected at a more recent date, the whole series of rocks forming a complicated network.

Again, in Dunrossness, between Quendale Bay and Loeh Spiggie, there is a mass of intrusive rock termed by Hibbert epidotic syenite, which is traceable northwards through the islands of Oxna, Hildasay, the Sandistura rocks, the Channes, and part of Papa west of Scalloway, to the Mainland in Bixetter Voe and onwards to Aith Voe. This rock varies considerably in character throughout its course; in some places it is a quartz-felsite, while in the neighbourhood of Bixetter and Aith Voes it is a true porphyritic granite, with large crystals of orthoclase. There can be no doubt that it is an intrusive mass, because it crosses obliquely the strike of the metamorphic rocks on Fitful Head and the Wart of Skewsburgh; and it is equally clear that the eruption was prior to the Old-Bed-Sandstone period, as the basement breccias of that formation rest unconformably on this rock, and are largely made up of angular fragments of the subjacent mass.

A similar mass of porphyritic granite occurs in Unst on the bluff headland of Lambaness and on the rocky promontory north of Skaw Bay, which likewise bears important testimony regarding the direction of the ice-movement. In addition to these masses there are minor veins of granite, gabbro, and serpentine, some of which are indicated on the map. There is one fact bearing on the age of the veins of serpentine on the Mainland which is worthy of note; and that is, the occurrence of fragments of this rock in the basement breccias of the Old Red Sandstone in Dunrossness. This circumstance plainly indicates that the formation of the serpentine veins in that neighbourhood preceded the formation of the breccias.