The Old Red Sandstone.

A glance at the map will show the various areas occupied by the members of this formation in Shetland. Beginning with the irregular areas on the east side of the Mainland, the succession may be most readily grasped by means of the following section (fig. 1, p. 784)^*.

Owing to a scries of faults which form the boundary-line between the metamorphic rocks and the Old Red Sandstone, over a great part of the districts of Lerwick, Quarff, Conningsburgh, and Dunrossness, it so happens that different zones in this vertical section are brought into conjunction with the schistose rocks. The true base of the series, however, is exposed in the neighbourhood of page 784

Fig. 1.—Vertical Section of Old Red Sandstone strata on east side of Shetland.

East Quarff, on the hills to the north of the bay and to the south towards Fladabister; while still another locality is met with near Loch Spiggie in Dunrossness. In each of these localities the breccia varies in character according to the nature of the underlying rock.

In the bay west of Brenista Ness, the overlying series of the Brenista Flags is thrown against the breccias and underlying schists by a fault which is traceable inland in a N.N.W. direction. This series consists of well-bedded rod flags, which persistently dip to the east till Gulberwick Bay is reached. The fault just referred to, when traced inland, always throws the flags down against the basement-breccia, and hence the actual superposition is not satisfactorily seen in the neighbourhood of Brenista. Between East Quarff and Fladabister, however, the one group may be seen resting conformably on the other; and, in addition to this, we find that the basal breccia, which forms vertical cliffs on the coast-line about 200 feet high, thins out inland till there is only about 3 feet of breccia between the underlying schists and the overlying Brenista Flags. In some instances the breccia disappears altogether, and the Brenista Flags rest directly on the schists, a fact which points to the gradual submergence of the area.

Returning to the shore-section north of East Quarff, there is a gradually ascending series from the Brenista Flags to certain coarse conglomerates seen in a small stream at the head of the bay of Gulberwick, which are totally different from the basal breccias already described. Not only are the enclosed pebbles well rounded, but to a large extent the stones are composed of different materials. These beds are traceable up the slope of the Gulberwick hollow, to the road between Lerwick and Scalloway, where they form crags on the hill face, and where they may be seen in small quarries by the roadside. They may be followed also across the hills northwards to Rovey Head, about two miles north of Lerwick, where they are brought into conjunction with the schists by a fault which is well seen on the shore. From Rovey Head southwards to the ridge overlooking the head of Fitch Dale, this fault forms the boundary- page 785 line between the conglomerates and the metamorphic rocks. At this point it dies out, and the boundary-line southwards towards Fladabister is formed by the basement breccia already described.

At Rovey Head the conglomerates are thrown into synclinal and anticlinal folds; but eventually they dip to the south-east, and are succeeded immediately by grey sandstones, with blue and grey flags passing upwards into the series of the Lerwick Sandstones. The dominating members of this series are coarse grits, frequently conglomeratic, with partings of fine red shales.

In Bressay, however, these arenaceous and conglomeratic strata are overlain by a more flaggy series, which is more or less persistent till Noss Head is reached. We were struck with the resemblance which some of these grey flaggy bands bear to the calcareous flags of Orkney and Caithness containing the fish-remains; but a careful search failed to bring any to light. Numerous plant-remains have long ago been detected, not only in these strata but also in some of the other groups on the eastern shore of the Mainland.

In the peninsular tract of country which lies to the west of the Weesdale district there is a great series of rocks which, with the exception of a small tract at Melby, have been hitherto considered as forming part of the metamorphic series. The small strip of Old-Red-Sandstone rocks at Melby, measuring about a mile and a half in length, has been referred to by previous observers. They are separated by a fault from the red quartzites and shales of Sandness Hill; and on approaching the fault it is observable that the beds are much shattered on account of this dislocation. They consist of reddish sandstones with dark blue flags and shales, dipping to the east of south and south-east, from Sandness to near Melby.

The great series of rocks which occupies almost the whole of the remainder of this peninsular tract, and which by their fossil contents we have proved to be of Old-Red-Sandstone age, has a somewhat different lithological character. Over a great part of this area the beds consist of grey and blue altered sandstones, with green and pale shales. The altered sandstones are usually traversed in every direction by joints, which are coated with peroxide of iron; and in places the beds have a marked schistose character. Sometimes the sandstones are converted into genuine quartzites, and the shales inter-bedded with them are distinctly cleaved. The strata lie in a trough the axis of which runs approximately from Fontabrough Voe eastwards by the village of Walls to the head of Bixetter Voe. On the north side of the syncline we have a gradually ascending series exposed on the coast-lino from the cliffs of Sandness Hill southwards towards Fontabrough Voe, the average strike of the beds being E. 20° N.

We discovered the plant-remains on the hills north of Walls, and subsequently in quarries by the roadside east of the village, and on the hills between Gruting and Bixetter Voes. They have been examined by Mr. C. W. Peach, who has kindly furnished the notes on the specimens embodied in the Appendix. He is of opinion that the plants are identical with those found in the Old-Red formation of page 786 Caithness and Orkney; and the strata in which they are imbedded, altered though they be, must be relegated to that period.

This conclusion is still further strengthened by the occurrence in these rocks of interbedded porphyrites and tuffs in a highly altered form, which we detected on the headlands between Aith Ness and Clouster, and on the western shore south of Dales Voe, resembling in many respects the contemporaneous volcanic rocks to be described presently. Further, we are inclined to believe that the series of altered thick-bedded sandstones and shales which occupy the greater portion of this peninsular tract are on the same horizon with the Lerwick Sandstones on the eastern side of the Mainland.

It is not improbable that the alteration of the strata in this wide area may be duo to the existence of a mass of granite underneath these rocks. We shall have occasion to refer to the mass of granite in the heart of these beds in Sandsting, and to similar intrusive masses of Old-Red-Sandstone age to the north. The extent of ground occupied by these acidic rocks indicates the great volcanic activity which prevailed during that period; and though these are now isolated at the surface, it is highly probable that they may be connected underneath.

These altered fossiliferous strata are brought into conjunction with the gneissose rocks to the cast and north by two great faults which we have traced on the ground, the one running north and south, and the other approximately cast and west. Usually the altered strata are terribly shattered and baked close to the lines of dislocation, and are likewise injected with numerous veins of very fine-grained felsite.