Extract

Among the various rocks of Scandinavian origin, boulders of which occur in our east-coast boulder-clays, and have been utilised by Glacial geologists in tracing the movements of the Pleistocene ice, none are better adapted for this purpose than the rhomb-porphyries. Not only have they a striking and easily-recognized characteristic in the peculiar crystallographic habit of their porphyritic felspars, which causes them to present rhombic sections on a face of the rock, but in this and other peculiarities they are unique or almost unique* among known rocks. They are thus among the few boulders which can be set down at a glance and without fear of error as having their source in a defined foreign area.

Although this conclusion seems beyond doubt, it may, perhaps, be thought to be strengthened by the cumulative evidence derived from the identification among our boulders, not only of rhomb-porphyries, but of particular varieties of rhomb-porphyries, which have been distinguished as such in their Norwegian home. The occurrence of rhomb-porphyry boulders at Bridlington, Dimlington, and other places on or near our east coast has long been known, but nothing more than a cursory notice of them has yet appeared. For a full description and comparison we must wait until the several types have been more completely and systematically described by the Norwegian geologists; the only object of the present notice is to point out that the Holderness boulders comprise more than one of the types already known in Norway.

Brögger, in his work on the eruptive …