Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/400

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Thus closely are we enabled to track the course of the forms which disappeared for a time from our seas and then returned. A knowledge of the geology of Spain and Portugal, as yet but very partially explored, may show hereafter that their place of refuge was not so far distant as the region into which we have followed them. The currents from the south sweeping the Lusitanian shores, and impinging over our own coasts—as Rennell's current for example—were, probably, and are still, powerful agents in the diffusion of the species, and in bringing about the present mixed condition of our marine fauna. In like manner, currents from the north probably brought back some of the sub-arctic forms so characteristic of the drift. But at certain spots within our area we find assemblages of northern forms so peculiar and so isolated, boreal patches, as it were, that we cannot account for them by any facts connected with the present disposition of the currents, or other transporting influence. These "patches" are especially to be met with in the Clyde district, and among the Hebrides, where they have been explored by Mr. MacAndrew; they have also been observed by Captain Otter, R. N., of Her Majesty's surveying ship Sparrow, on the east coast in the Murray Firth. It is probable there is a similar patch somewhere near the Nymph Bank on the east coast of Ireland, and another in the German Ocean. They have, usually, for their centre, a hole or valley of considerable depth, from 80 to beyond 100 fathoms. Their inhabitants are decidedly of more northern character than the members of the