Contemporaneous Igneous Rocks.

In the western district of Northmavine, between Stennis and Ockren Head, at the mouth of Roeness Voe, there is an important development of ancient lavas and ashes, associated at certain localities with ashy sandstones and red flags belonging to this period. The tract of ground occupied by these rocks measures about six miles in length, and varies in breadth from one to two miles. The structure of this narrow tract is comparatively simple, as the strata form a flat syncline, in the centre of which lies coarse ash, and underneath a series of slaggy porphyrites with occasional beds of red ashy sandstones and flags.

On the south bank of Roeness Voe, rather more than a mile from Ockren Head, in a steep grassy goo, the slaggy porphyrites are brought into conjunction with the pink quartz-felsite by a fault. In Braewick Bay, west of Hillswick, the interbedded and intrusive igneous rocks are not found in such close proximity, the junction being concealed by the sandy beach; but there can be little doubt that the same fault runs out to sea in this bay.

Crossing the coarse volcanic breccia, which forms the centre of the syncline, to Ockren Head, at the mouth of Roeness Voe, the successive lava-flows are admirably shown, piled on each other in regular succession. This headland, as well as the adjacent stack, exhibit at page 787 least four different lava-flows, capped by coarse ash. The lavas thicken and thin out rapidly, and likewise exhibit. The usual slaggy structure at the top and bottom of the flow. Some of these beds are also highly involved, and show clearly the way in which the partially solidified crust has been caught up and rolled over and over in the advancing current of still molten lava.

We have already alluded to the porphyrites and tuffs which occur in the altered rocks north of Walls. We also detected a bed of lava in the Holm of Melby, and a thin bed of tuff associated with the grey flags on the east side of Bressay, opposite the north end of the island of Noss. The contemporaneous volcanic rocks found in Papa Stour have been previously described by Professor Geikie.