RECORD: Malcolmson, J. G. 1838. On the occurrence of Wealden strata at Linksfield, near Elgin; on the remains of fishes in the Old Red Sandstone of that neighbourhood; and on raised beaches along the adjacent coast. Proceedings of the Geological Society of London 2: 667-669.
REVISION HISTORY: Scanned by John van Wyhe, transcribed (single key) by AEL Data. RN1
A notice "On the Occurrence of Wealden strata at Linksfield, near Elgin; on the Remains of Fishes in the Old Red Sandstone of that neighbourhood; and on raised beaches along the adjacent coast;" by J. Malcolmson, Esq., F.G.S., was then read.
The country around Elgin is composed of sandstones, conglomerates, and concretionary limestones, belonging to the old red sand-stone; but at Linksfield, one mile south of Elgin, that formation is overlaid, unconformably, by a series of beds, which Mr. Malcolmson has ascertained, by their organic remains, to represent the Wealden strata of England, though they have been usually considered to he lias.
The following section gives the principal beds in descending order, the average thickness of the whole series being from 20 to 30 feet:
1. Blue clay, containing thin bands of limestone, the lower being shelly.
2. Thin bands of limestone and clay.
3. Blackish shale, not bituminous, 1 to 2 feet.
4. Compact grey limestone, without shells, in layers separated by clay, 4 feet.
5. Laminated green clay, with a network of fibrous carbonate of lime.
6. Red, sandy, calcareous marl, abounding with rolled pebbles of granite, gneiss, &c., also angular fragments of the fine-grained yellow and grey sandstone forming the hills to the west, but the geological position of which is not yet ascertained.
Cornstone of the old red sandstone in unconformable position.
The fossils are principally found in the lower bands of the top bed. They are rarely well preserved, and cannot be separated from the rock. The species are few in number; but abundant in individuals; and one species of Cyclas is indistinguishable from the C. media of Sussex, found also by Prof. Sedgwick and Mr. Murchison in the Isle of Skye: there is likewise an Avicula, which agrees with one found in the Purbeck strata at Swanwich. Mr. Malcolmson procured also fragments of an Astarte and a Venus, and a microscopic univalve. 'The clay below this shelly limestone is full of the valves of a new, round species of Cypris. The author also obtained teeth and scales of fishes; and the Rev. G. Gordon has found a Saurian bone.
Fossils of the same description have been recently discovered by that gentleman at Lhanbryde, three miles to the east of Linksfield; and in a micaceous white sandstone, he has procured a large Pinna, which Mr. James Sowerby has identified with a species found in the Portland sand of England. In April, 1832, Mr. Gordon communicated to the Society a notice of the discovery in a dark clay*, penetrated while draining the Lake of Spynie, of the Turritella muricata of the Coral Rag. Mr. Malcolmson, therefore, hopes that many members of the series above the old red sandstone, not yet known to exist south of the Murray Frith, will be discovered by the practical geologists resident in that district.
Mr. Martin, of the Anderson Institution, has recently discovered in a bed of calciferous conglomerate, near Elgin, and supposed by Mr. Gordon to represent the old red sandstone of Clasbennie in
* Proceedings, vol. i. p. 391.
Perthshire, scales, teeth, and bones of fishes; and, by comparing these remains with a magnificent specimen of a fish from Clasbennie, in Mr. Murchison's possession, Mr. Malcolmson has ascertained this supposition to be correct. A doubt, therefore, which formerly existed respecting the age of the conglomerate, is now removed.
The paper concluded with an account of eleven ancient beaches on the coast, rising above each other, and from one of which, 15 feet above high-water mark, and cut through in draining Loch Spynie, Mr. Malcolmson procured twelve species of existing marine tacea.
A paper, "On the Origin of the Limestones of Devonshire," by Robert Alfred Cloyne Austen, Esq., F.G.S., was afterwards read.
The object of the paper is not to account for the origin of calcareous matter, or the means by which marine animals derive it from the surrounding medium, but to show how far the limestones of South Devonshire may have been produced by polypi.
These limestones are stated by the author to occur, in nearly every instance, in the immediate vicinity of volcanic disturbances, and to be partly included in the slates and sandstones, and partly to rest upon them. To the former belong the broad band extending from Staple Hill to Dean Prior, the minor bands in the neighbourhood of Hempstone and Totness, and all those which occur beyond the Dart; also the limestones of Newton and Torbay. They are said to be less pure and more slaty than the overlying limestones, and to be frequently separated by seams of shale. Transverse sections of these bands show, that the strata in some cases become thinner as they descend, and that the partings of shale increase, as near Staverton in the valley of the Dart, and at Staple Hill; but that in other instances, as between Newton and Totness, the strata instead of fining off end abruptly upon the slate, and are covered in the direction of the dip by similar slates. The strata are always inclined, but they invariably form a table-land at the surface. This inclined position the author conceives is not due to dislocation, but to the beds having been deposited a the angle which they now present; and he illustrated his opinion, by a section between three and four miles in length, through the parishes of Pegwell, Denbury, and Abbots Kerswell, a remarkably level country. The bands of limestone dip 40°, but are nowhere more than 150 feet thick, and they all contain the same decription of organic remains. If the bands were deposited horizontally, and the most recent nearly at a level with the surface of the ancient ocean, then the lower beds, the author says, would have been placed at a depth of nearly three miles, although the organic remains prove that all the beds were formed under precisely similar conditions.
In the structure of the Devonshire limestones, however, Mr. Austen considers that he has discovered evidences of an origin similar to that of modern coral reefs, and which will explain their inclined position. At Ogwell Park the limestone forms a horizontal capping to the inclined strata; and at Bradley rests conformably against a ridge of slate,
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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