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II.—Notes on the Geology of the Neighbourhood of Aberystwyth

Published online by Cambridge University Press:  01 May 2009

Walter Keeping
Walter Keeping
Affiliation:
Professor of Geology in the University College of Wales.
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Extract

In England and Wales there is scarcely any area whose geological structure is so little known as the neighbourhood of Aberystwyth. Every year a stream of geologists passes through the northern parts of the Principality, through the varied old-volcanic districts of Snowdon and Cader Idris, and the rich collecting grounds of the Berwyns: and another such stream takes its course to the southern borders along the far west Pembrokeshire coasts, being guided by the careful work of Dr. Hicks. The border counties too are in many places attractive enough, for there it was that Murchison first saw the order of the Silurian rocks, and there also the best-preserved fossils are found. Llandeilo, Builth, Woolhope, May Hill, Malvern, and Wenlock have long yielded rich harvests to palæontologists.

Type
Original Articles
Information
Geological Magazine , Volume 5 , Issue 12 , December 1878 , pp. 532 - 547
Copyright
Copyright © Cambridge University Press 1878

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References

page 534 note 1 This Foraminifer was first discovered by the Rev. Blake, J. F. M.A., F.G.S., See Geol. Mag. Dec. II. Vol. III. p. 134.Google Scholar

page 535 note 1 It should be noted that these marvellous surface contortions and other markings, so characteristic of the Mid Wales rocks, are convex on the under surface of the flags and grits.

page 535 note 2 Mostly seaweeds.

page 536 note 1 This contorted structure forcibly brings to mind the irregular contortions which are so generally found in foliated rocks, such as mica-schist. The same cause may well have produced the structure in both rocks.

page 536 note 2 The great apparent thickness of a group of rocks in the Moffat District of South Scotland has been explained by Mr. Lapworth as produced by frequent sharp foldings, by which the beds are repeated over and over again. But we have no evidence that such an explanation can be applied to the rocks between Aberystwyth and Plynlimmon.

page 537 note 1 N.B.—These rocks are very commonly known as Silurian. Through a series of errors, Sir Roderick Murchison felt compelled to absorb Sedgwick's Cambrian almost entirely into his Silurian System. But so good and true a name as the “Cambrian System,” as applied to a great and natural group of rocks, honestly and successfully worked out, must not be allowed to be degraded into the appellation of a small part of that system. Such a course is not only unfair to our great leader in Cambrian geology, but is also, as I believe, a hindrance to geological science. As a name, too, the change would be unfortunate.

page 538 note 1 The rock structure we have been describing above is known as cleavage, and a slate is the result. Shale is a rock which splits into thin layers along planes produced by the circumstances of its deposition. As a rule, slate is harder and more regular than shale, and especially slates often split across the planes of bedding; shale rarely does so.

page 539 note 1 It is remarkable how very frequently a lead mine is found at the head of a valley, and again they are frequent along the sides of valleys. This is probably due to the metalliferous veins, being lines of comparative weakness, so determining the lines of action of subaerial denuding forces as to produce valleys.

page 541 note 1 All these blocks are from the local grit and hard shale-beds, or more rarely a conglomerate from Plynlimmon. In the Ystwyth Valley, and towards Machynlleth, a pale-coloured hard granular and felsitic-looking rock occurs, whose origin is not yet known to me.

page 541 note 2 It seems probable, from subsequent observations, that this local name Esgair, may sometimes have been applied to mounds of a different origin and structure from that of Eskers properly so called.

page 542 note 1 One of these was presented to the University College Museum by Mrs. Davies of Antaran.

page 543 note 1 The name of a farm-house near Penllwyn.

page 545 note 1 The depths of many of the lakes are not yet known to me.

page 545 note 2 The visitor must take care to distinguish the natural lakes from the artificial reservoirs of water made by the lead-workers to serve for a steady supply of water to their water-wheels.