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The Pamphlet Collection of Sir Robert Stout: Volume 22

Intrusive Igneous Rocks

Intrusive Igneous Rocks.

In the north and western portions of the Mainland there is a splendid development of highly siliceous intrusive rocks, which occupy the most elevated ground in the island. They extend from a point on the north end of the Mainland opposite the island of Uya, southwards to Roeness Voe, culminating in the dome-shaped mass of Roeness Hill. Thence they cross the peninsular tract to the Heads of Grocken, west of Hillswick, reappearing in the slender columns of the Drongs. The western portion of Meikle Rooe is formed of the same material, and likewise the north-eastern headlands of Vemeutry, while the small area of quartz-porphyry at Melby must also be included in the same great intrusive series. In addition to the areas now referred to, there are other lenticular masses varying in size down to veins a few feet across, occurring at intervals from Mavis Grind, northwards to Roeness Voe and Ollaborry. These traverse the Northmavine diorite and metamorphic rocks alike, increasing in number and extent as they approach the Roeness mass.

These rocks vary considerably in character; but they all agree in possessing a largo proportion of silica, while the felspar is usually orthoclase. As a rule, they are coarsely crystalline, the two prevalent ingredients, quartz and orthoclase felspar, being distinctly crystallized, which causes the rock to assume a marked granitoid texture. There can be little doubt that these coarsely crystalline rocks must have originally consolidated under great pressure, though the materials under which they lay buried have been wholly removed by denudation. Further, the marked columnar structure which meets the eye along both banks of Roeness Voe, and from the Heads of Grocken to Braewick Bay, as well as along the western shores of Meikle Rooe, suggest the idea of a great intrusive sheet, forced in like a wedge between the metamorphic series and the members of the Old Bed Sandstone long since worn away. A similar intrusive sheet occurs in Papa Stour, as described by Professor Geikie, where the same columnar structure is apparent, and where a fragment of the once superincumbent strata is still to be seen at the Horn of Papa. Fortunately the intrusive nature of this latter shoet is placed beyond doubt, inasmuch as the pink porphyry is seen page 788
W. Ockren Haad Roeness Voe. Roencess Hill The Biurgs. 1. Metamorphic rocks. 2. Pink granite and quartz-felsite. 3. Bedded porphyrites and ashes. 4. Serpentine.

Fig. 2. Section across Northmavine from Ockren Head to Skea Ness.

cutting across the underlying sandstones from a lower to a higher horizon.

As the result of careful mapping of the boundaries of the Northmavine mass, we are of opinion that the Roe-ness-hill plateau is a great intrusive sheet which forced its way upwards and laterally between the metamorphic strata on the one hand, and the members of the Old Red Sandstone on the other, at the time when the Mainland lay buried under the sedimentary deposits which accumulated during that period. It is not at all improbable that this immense mass may have been connected with the surface by pipes which traversed the superincumbent strata, and may have discharged volcanic materials at the surface.

Its relations to the metamorphic series are admirably defined. Along the eastern seaboard of Northmavine it forms a mural escarpment about 200 feet high, part of which is known by the name of the Biurgs. Innumerable veins of quartz-felsite branch off from the main mass and intersect the metamorphic series. Further, it sometimes happens that portions of the adjacent rocks are enclosed in the quartz-felsite, as, for instance, near Colifirth Voe, where a fragment of serpentine is caught up in the mass. Again, on the north bank of Roeness Voe, the sheet spreads over the edges of the diorite and metamorphic rocks without producing any deflection of the strike of the metamorphic series, as shown in the accompanying section (fig. 2).

To the west of Hillswick, on the picturesque Heads of Grocken, the highly siliceous quartz-felsite is thrown against the metamorphic series by a fault, which is well seen on the cliffs. This fault passes out to sea between the little islets of Waterhouse Holm and the Drongs, and reappears in Meikle Rooe, separating the quartz-felsite from the diorite of that island. In all likelihood this dislocation is the northward prolongation of the great north-and-south fault already described, which brings the metamorphosed Old-Red-Sundstone rocks into conjunction with the gneissose rocks of Weesdale.

page 789

Our conclusion regarding the Roeness plateau is strengthened by a consideration of the relations of the granite mass of Sandsting to the altered Old-Red strata of that district. This siliceous intrusive rock has several lithological varieties; but away from the margin of the area, as, for instance, on the hills above Gruting and round Skelda Voe, it is an ordinary granite consisting of pink orthoclase, quartz, and mica.

On the shores of Gruting Voe, at the foot of Culswick Hill, the junction of the granite with the Old-Red quartzites and shales is well seen, from an examination of which it is evident that the granite is intruded along the lines of bedding of these strata. The junction-line has nearly the same inclination as that of the quartzites, which dip to the north at an angle of about 20°. The two rocks are not strictly conformable, however; for the granite here and there cuts across the bedding, indicating in an unmistakable manner the intrusive nature of the igneous rock. The junction is a sharp and well-defined line, as small hand-specimens can easily be got, 2 inches across, including the granite and the quartzite, the two being firmly welded together. Near the junction of the two rocks the quartzites are pervaded by numerous dykes of pink felstone proceeding from the main mass.

The mass of granite in the north of Delting and on the western shores of Sulem Voe was probably erupted during the same period of volcanic activity; but the evidence is not so convincing as that referring to the areas already described.

But in addition to the grand series of intrusive rocks we have just indicated, there is evidence to prove that even these quartz-felsites and granites were invaded by a still later series of dykes, of a basic character. Hibbert detected the existence of these dykes on Roeness Hill; and during our traverses in the district of Northmavine, Delting, and Meikle Rooe we came across many similar masses, varying in breadth from 2 feet to several yards. Along the cliffs of Roeness Voe, and in the island of Meikle Rooe, these dykes are strikingly exhibited, forming great wall-like masses, running generally in a north-and-south direction. Sometimes they project above the acidic rocks, while, again, they weather more rapidly, forming great clefts in the face of the cliff. They are fine-grained, and consist of a dark-green diabase porphyrite. They traverse the metamorphic rocks, as well as the porphyrites and tuffs, west of Braewick; and there can be little doubt, therefore, that they form the last indications of volcanic activity during the Old-Red-Sandstone period in Shetland.

Close by the entrance to the Noss Sound, on the Bressay shore, we detected a scries of necks arranged in a linear manner, which seem to have come to the surface along a line of fissure. Similar necks occur on Noss, on the opposite side of the Sound. It is highly probable that these volcanic orifices served merely as vents for the discharge of steam, with occasional showers of triturated materials derived mainly from the sides of the vents. The adjacent bed of tuff, associated with the grey flags, as well as the nature of page 790 the agglomerate which now fills these necks, seems to support this view.