The Old Red Sandstone of the Orkneys

• Main Author: Flett, John Smith
• Published: University of Edinburgh 1900
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734975

It will be seen that in the Orkneys we have one of the most abundant and interest? ing series of the camptonite-monchiquite-alnoite rocks which is anywhere known to exist. They fall naturally into two groups, the " leucocrate " rocks of Brogger, of which the sole representative is one dyke of bostonite, and the " melanocrate," to which the others belong. Between them there is no connecting link; and the theory which he expounds of their origin by complementary differentiation from the same magma, is, in view of their constant association, the most natural that has been suggested. The " melanocrate " series must be regarded as a unity. The series of gradations by which they merge into one another is so complete that they cannot be separated. They have proceeded from one magma, which, by a progressive differentiation, was becoming more and more basic as time elapsed. Taking the Orkney dykes as a whole, the petrographical characters agree with the facts regarding their geological occurrence, in establishing that they have been emitted, no doubt in successive periods, from the same volcanic focus. But when we pass to consider what the nature of that parent magma has been, it must be insisted that there are no data, within the Orkneys, which will enable us. to decide. In chemical composition they correspond exactly to no plutonic magma, and seem to be in every case products of differentiation. They are typical dyke rocks, and occur only as dykes or thin intrusive sheets. It may be possible, when their distribu? tion in the north-east of Scotland is better known, to trace them to some parent mass ; till then we can argue only from general facts. From what is known of this series at the present day, it is difficult to point to any magma from which we could say with certainty they had not been derived. Granites, augite-syenites, elaeolite-syenites, gabbros and diabases, and tberalites, have all been shown to have occasionally given origin to dykes of one or other of the rocks of this series. In the present instance only one fact seems to point to a solution of the problem. In Orkney the camptonites are the most numerous, and form a central type from which the others diverge. Of these there is only one dyke, though it is connected with the others by intermediate steps, which corresponds chemically and structurally to the products of undifferentiated magmas. That is a diabase, and we may provisionally regard it as most probable that the original magma was, as Brogger has shown for the camptonites of Gran, an olivinegabbro- diabase magma. If so, it may well be, as Sir Archibald Geikie suggests, that they are outlying members of the great Tertiary series of basaltic dykes of the west of Scotland.