Abstract
II. III.—The Silurian Period. AFTER the long interval of time represented by the elevation of the red sandstones into dry land, and their entire removal from some places by denudation, the north-west of Scotland, and probably a large tract lying around it, sank under the sea. The depression seems to have been slow and gradual, and to have continued until the site of the Cambrian basins and of the surrounding region was covered with a considerable depth of clear, open sea-water. The records of this subsidence are contained in a series of strata having a total thickness of somewhere about 2000 feet, and divisible into two chief groups—a Lower, composed of quartzites, grits, and thin conglomerate, about 500 feet in total depth, and an Upper, consisting almost wholly of limestone. Perhaps the most striking feature in this series of stratified rocks is the abundance of their organic remains. The quartzites are crowded with the tubes formed by sea-worms when the material existed as soft white sand on the sea-bottom. The limestones are made up of the remains of calcareous organisms, among which the most conspicuous that now remain are chambered shells and Gasteropods. Throughout these limestones, worm-casts are present almost everywhere, and in such abundance as to show, as Mr. Peach has pointed out, that "nearly every particle of the calcareous mud must have passed through the intestines of worms."A large collection of fossils has been made by the Geological Survey from these limestones, which, though not yet specifically determined, amply confirm the original generalization of Salter, made more than thirty years ago, that the aspect or facies of organic remains in the limestones of the north-west of Scotland resembles that of the older parts of the Lower Silurian formations of Canada rather than that of the corresponding rocks in Wales. So marked is the resemblance to the American type as to indicate that some shore-line must once have stretched across the North Atlantic, in order to afford a platform for the free migration of marine life between the two areas. The contrast with the Welsh type has been explained by the probable existence of some barrier that separated the sea-bed over the north-west of Scotland from that of Southern Scotland, England, and Wales. That such a barrier existed is tolerably certain, and I shall presently refer to some indications of its probable position. At the same time it may be open to question whether the Durness limestones can be properly correlated as homotaxial equivalents of any Lower Silurian rocks in Wales. My own impression is that they.may be older than the oldest Arenig rocks, and may be equivalent to some part of the "Primordial Silurian"or Cambrian series. This, however, is a question that must remain unsettled until a thorough critical examination of the fossils has been completed.
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Recent Researches into the Origin and Age of the Highlands of Scotland and the, West of Ireland. Nature 40, 320–324 (1889). https://doi.org/10.1038/040320b0
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DOI: https://doi.org/10.1038/040320b0