As the distribution of the rock-formations has an important bearing on the question of the glaciation, it will be desirable to give a brief outline of the nature and respective limits of the various formations, so far as these have been already determined. The stratified rocks belong to two periods:—(a) the Old Red Sandstone; (b) the great series of metamorphic crystalline rocks on which the representatives of the Old Red Sandstone rest unconformably. To what precise part of the crystalline rocks of the Highlands the metamorphic series of the Mainland and the north isles belongs, we do not at present presume to say^*.

There are also associated with the metamorphic series some intrusive igneous rocks, and certain masses which may be viewed as products of extreme metamorphism. These may probably be relegated to the time when the metamorphism of the ancient stratified rocks took place. At least some of the igneous rocks now referred to must be older than the basement breccias of the Old Red Sandstone, inasmuch as the latter in certain localities are composed of angular fragments of the former.

But, farther, there are abundant proofs of volcanic activity during the Old-Red-Sandstone period, as is evident from the great development of contemporaneous and intrusive igneous rocks on the Mainland. Similar phenomena are met with in the isles of Papa Stour, Bressay, Noss, the Holm of Melby, and Meikle Rooe; but the magnificent sections on the western shores of Northmavine justify the conclusion that the proofs of volcanic activity on the Mainland surpass in grandeur and extent those of the other Shetland islands.