Transactions of the Geological Society, 1st series, vol. 4/On Magnesian Breccia
|←On the Strata in the Neighbourhood of Bristol||On Magnesian Breccia
|On the Magnesian Limestone of the neighbourhood of Bristol→|
By HENRY WARBURTON, Esq.
vice president of the geological society.
[Read 21st June, 1816.]
THE great stratum of magnesian limestone which passes from Sunderland in the north of England through the centre of the midland counties, suddenly terminates, as is well known, in the vicinity of Nottingham; and I am not aware of its reappearance in the south of England having been noticed except perhaps on the north-eastern border of the Ashby de la Zouch coal-field, where it is said to occur in great insulated masses.
The geological relations of this rock to other strata. appear to have been well ascertained in the northern and midland counties, where it is described as forming horizontal beds, and as lying under and parallel to the red marl, or occasionally as alternating with it. It has been ascertained by numerous sinkings in the same counties that the magnesian limestone lies over the coal measures; it is doubtful however whether the coal measures are conformable with the strata of the magnesian limestone; and it is not improbable that they lie under it, having the edges of their tilted and broken sills abutting against the lower surface of the superincumbent rock.
The red marl is so widely distributed in that part of England which lies between Lancashire and the southern coast of Devonshire, and is so frequently found in that district in the same geological position which it occupies in the northern and midland counties in alliance with the magnesian limestone (lying for instance in horizontal strata upon the inclined coal measures, and bounding them at their basset) that it might be expected in some part of its course to discover traces of the magnesian rock. Accordingly I shall mention some instances of the occurrence of a magnesian limestone in the district above referred to, where it either alternates with red marl, or may be considered as connected with it.
In the course of a valuable paper on the Rocks in the vicinity of Bristol, which was long ago presented to this Society, the author, Mr. Bright, has given an account of the strata of red marl which lie along the banks of the Avon. The red marl is there found either lying upon the coal measures, or filling up the vallies that are occasioned by the breaking off of the inclined strata of limestone, where instead of the series of inclined strata that should rise from beneath the limestone, horizontal strata of red marl are found resting upon the broken edges of the limestone or of the first of the rocks beneath it. It is in the red marl last described, as it occurs near Hung-road on the Avon, that Mr. Bright discovered a limestone breccia, of which there are two beds alternating with red marl.
Having examined this breccia on the spot, after having consulted Mr. Bright's paper, and having seen some breccias from the Mendip Hills of which I knew the nature, and which very much resembled those from the banks of the Avon, I had no difficulty in ascertaining that the cement of the latter was composed of magnésian limestone; of which indeed the characters are so strongly marked as to be evident on mere inspection. This breccia consisted of rolled fragments of milk white quartz, and of angular fragments of limestone and sandstone such as are found in the neighbouring inclined strata cemented together by yellow magnesian carbonate of lime; the cement being in great excess. I must refer to Mr. Bright's paper (which I understand will soon be published) for the further description of this rock.
Shortly before seeing the rocks of Hung-road, I had been with the late Smithson Tennant, Esq. to examine the magnesian breccia which he had observed on the Mendip Hills near to the celebrated cliffs of Cheddar. The southern declivity of that limestone chain is there deeply furrowed by wide and extensive combs, in which immense blocks of the breccia many yards in diameter are found lying upon the surface of the limestone. The strata of limestone dip to the south under an alluvial valley, by which they are separated from a low chain of red marl that is found at the distance of about half a mile to the south.
The breccia of the Mendip Hills very much resembles the breccia from the Avon, consisting of fragments of limestone, magnesian limestone, and sandstone cemented together by a yellow magnesia carbonate of lime; but I never discovered in it any of the quartz pebbles which are imbedded in the breccia from the Avon.
Until I had seen the breccia at Hung-road I was unable to account for the presence of these immense insulated blocks upon the sides of the Mendip; but I have since ventured to conjecture that they once formed a subordinate bed in the strata of red marl which are found on the other side of the alluvial valley of the river Axe, and which perhaps were originally continuous across the valley and rested mediately or immediately upon the limestone; but which have since been removed by denudating causes, the hardest and most durable part of their mass, the magnesian limestone, being left behind.
I have heard of the following additional cases in which a magnesian breccia is found in connection with red marl. Dr. Wollaston in the first instance and afterwards Mr. Greenough informed me that a similar rock was found near to Cowbridge in South Wales, a specimen of which was presented by the latter to the Society. Mr. Aikin also has noticed a breccia of the same description at Caerdeston and Loton in Shropshire.
In thus comparing the magnesian breccia of Bristol with the yellow limestone of the northern and midland counties, I have assumed that the red marl which lies above the coal measures is of the same order with that which lies at the bases of those escarpments, where strata of mountain limestone are broken off; and where instead of the lower beds rising from beneath the limestone we find horizontal strata of red marl filling the plains. I am not prepared to establish this by any positive proof; such evidence as the geology of the plain of Carlisle would afford is already in the hands of Mr. Buckland; the appearances that are to be sought after for determining this question, and which perhaps may be observed in the neighbourhood of Bristol, are the following: no disposition of the strata is more common in the country between Bristol and the Mendip than that described in Mr. Bright's paper; where a ridge of mountain limestone separates two plains from one another, each containing horizontal beds of red sandstone or marl, the one lying above the limestone stone with the intermedium (perhaps) of the coal measures, the other abutting against the broken edges of the strata of limestone at the base of its escarpment. Perhaps there may be found some valley of denudation connecting together the two plains, which being itself filled with red marl of the same description, there may be an uninterrupted bed of marl through the valley from one plain to the other.
The determining of this question would be of some importance as a matter of speculation, and of some practical consequence to the coal viewer. Those who consider the red marl as one of a complete series of beds succeeding one another in a uniform order, will in every case expect to find the coal measures on sinking through the red marl. If on the contrary we suppose denudatory or other disturbing causes to have been in action previously to the deposition of the red marl, we might expect to find the red marl immediately incumbent upon any rock from the coal measures to the granite inclusive, just as the alluvial beds in which the bones of elephants are found in consequence of previous denudation are discovered resting either upon the blue clay of London, upon the Oxford oolite, or any other bed: and on this view of the subject the red marl will no more be an indication of coal than of any other member of the lower strata.
- See his paper, page 105 of the present volume.