Transactions of the Geological Society, 1st series, vol. 4/On the Magnesian Limestone of the neighbourhood of Bristol

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On the Magnesian Limestone of the neighbourhood of Bristol
by William Hall Gilby

IX. On the Magnesian Limestone and Red Marl or Sandstone

of the neighbourhood of Bristol.

By W.K. Gilby, M.D.

[Read November 15th, 1816.]

THE present communication originates from my having discovered in this neighbourhood the magnesian limestone which exists so extensively in the North of England; and as its position with regard to the other formations is remarkably distinct, we have here at least an opportunity of ascertaining its true geological rank, concerning which much uncertainty has prevailed in other quarters.

In the description which I have given in the Philosophical Magazine of the geology of this neighbourhood, it will be seen that we have here two grand divisions of rocks, one occurring always in inclined, and another in horizontal strata. The lowest formation of the first class is the first flœtz or old red sandstone, exceedingly well characterized. It is only in certain tracks that this rock constitutes hilly ground so as to be visible to the eye: but the first flœtz or mountain limestone which rests upon it, has a very extensive range, describing in its course an irregular ellipsis, the direction of which it will here be unnecessary to repeat. I may however remark that analogous to what has been observed with regard to the ellipsis of mountain limestone in South Wales, the dip of its strata varies remarkably in different parts of its course. At the north the strata dip south, and at the south, north. On the east they have a western dip, and on the west an eastern one. In this way the strata tend every where towards a common centre. In the hollow of the basin so formed is deposited a very extensive coal formation, for the particulars of which I refer to the above description. It is sufficient to say that the coal beds and coal measures are always inclined, and when contiguous to the mountain limestone they always dip conformably with it.

It is upon the tops or edges of the inclined strata of these formations that we find the strata of the second division, that is, the horizontal rocks, superimposed in an unconformable position. These horizontal rocks are the same with those occupying so large a portion of the south and south-west of England. The lowest of them are the beds of the red ground, as this formation has been absurdly denominated, consisting of a coarse limestone conglomerate, above which lies a calcareous sandstone, red and white in different places, and then a deposit of red clay, containing gypsum and sulphate of strontian. Above this red ground formation occurs the well known lyas limestone, then the oolites or Bath stones, and lastly the chalk.

The basis of the red ground conglomerate I have generally found to be a common limestone; but being lately at a village on the Bristol Channel called Portishead, I was surprised to find the basis of the conglomerate of a yellow colour, and resembling in appearance some varieties of the Yorkshire magnesian limestone. Upon analysis I found that it did contain a considerable quantity of carbonate of magnesia, the proportion of which varies in specimens taken from different strata. In some strata the basis is so much mixed with sand as to give more than 20 per cent. of insoluble matter, consequently the quantity of carbonate of magnesia is much diminished. The fragmented portions are generally limestone or red sandstone, but we find some strata destitute of sand and fragments, forming in fact a hard compact magnesian limestone. This variety will give 36 or 38 per cent. of carbonate of magnesia. All the varieties are of a yellow colour, and like the magnesian limestone of the north, it often exhibits black spots throughout its substance, and it frequently contains impressions of shells.

The analysis of the compact variety, conducted in the usual way, gave me of

Carbonate of lime 53 .5
Carbonate of magnesia 37 .5
Oxyd of iron .8
Insoluble matter 7 .
Loss 1 .2
100 .0

Now with regard to its geological relations, it in no respect differs from the limestone conglomerate which I have mentioned as the lowest bed of the red ground formation. I have traced it in a continued line by the sea side from Portishead to Clevedon, and it every where contains the same fragments and every where lies horizontally and unconformable upon the inclined strata (which are there the old red sandstone,) in the same way that the usual red grained conglomerate does. It is therefore plainly to be considered as the lowest stratum of the red ground formation, and consequently succeeds immediately to the coal deposit.

It seems to me that the magnesian limestone of the north of England may be referred to the same formation. In Thomson's Annals there is lately a short notice, of a paper read before the Geological Society by Mr. Winch, upon the magnesian limestone in the north of England, in which it is stated that the Tees flows over beds of white and red calcareous sandstone containing gypsum, which rests upon magnesian limestone. Now this gypseous calcareous sandstone may be said to characterize the red ground, therefore the magnesian limestone in that district may be safely referred to that formation, and consequently may be regarded as the rock next in succession to the Durham coal deposit. This indeed might be collected from the imperfect sketch Dr. Thomson has given us of its occurrence in that quarter. In a late number of the Philosophical Magazine, it is said that the coal formation of Whitehaven is covered at St. Bees Head by bituminous clay, over this is a limestone containing magnesia and iron, and above this is a red sandstone, connected with which is clay, marl, and gypsum. This it will be perceived is almost an exact account of the succession of the red ground strata in this quarter. In Derbyshire we are informed that the magnesian limestone lies in an unconformable and horizontal position over the inclined strata, after the manner of its occurrence in this quarter, it therefore plainly belongs to the same formation, the red ground.

It may seem at first very remarkable that the basis of the limestone conglomerate should be in one place merely a common limestone, and that in another spot, not far distant, it should contain a considerable proportion of magnesia. To those however who have seen how widely the same rock formation, nay even the same stratum, will vary in its colour, hardness, and general structure in different parts of its course, this will cease to be a matter of wonder. In attempting to explain these singularities, it seems to me that we must resort to one of two suppositions: we must either conceive that the fluid menstruum during the deposition of any particular and extensive formation, must have contained in different places different chemical ingredients; or we must conclude that the alteration in structure in an individual formation, has not so much been derived from the addition or substraction of certain chemical ingredients as from the proportion in which these ingredients have crystallized. According to the latter view, during the consolidation of any particular formation, the constituent particles, although few in number, may in different parts of the crystallizing mass have been attracted together in new proportions, so as to give rise to those variations in colour and structure which we so frequently witness. The originality of this theory of crystallization belongs to Professor Jameson, and it seems to me very happily to explain many anomalous appearances of disorder and brecciated structure, which have caused great embarrassment to geologists. In some cases however this theory cannot be applied with any degree of probability. Where we see a particular assemblage of strata, as the limestone conglomerate, manifestly of the same formation, exhibiting in several parts of its extent changes of composition altogether depending upon a difference in its chemical constitution, it is impossible to explain such an occurrence but by supposing that the fluid menstruum must have contained in different places different chemical ingredients. Every geologist will figure to himself illustrations of the want of uniformity in the same rock formation. I may mention however two other striking facts of this nature. The red clay of the red ground is met with in almost every part of England, and almost every where does it contain or is connected with gypsum; but besides gypsum, in this neighbourhood only, it abounds with sulphate of strontian in the form of veins and even large beds. From Mr. Webster's account of the strata above the chalk in the Isle of Wight, it seems quite manifest that what he calls the first fresh water formation was formed at the same period with the marl and gypsum of the Paris basin, containing the bones of birds and fish, but an important difference is that the English strata are destitute of gypsum.

We sometimes meet with magnesian limestone subordinate to the first flœtz or mountain limestone. Sometime ago I discovered a very beautiful sparry dolomite lying in conformable strata upon the mountain limestone near Ross in Herefordshire. This variety contains 44 per cent. of carbonate of magnesia. A small ridge of rock, about four miles north-west of Bristol, upon which Lord de Clifford's house is built, is entirely composed of a magnesian limestone abounding in shells, entrochi, and madrepores; and in an adjoining hill which overlooks Blais Castle it occurs, as far as I can understand, interstratified with the mountain limestone. A specimen of this variety I find to be composed of

Carbonate of lime 58
Carbonate of magnesia 38
Oxyd of iron 1 .
Sillica and bituminous 1 .5
Loss 1 .5
100 .0

I may remark that this magnesian limestone varies remarkably in specimens taken even from contiguous situations, both in colour and other external characters. It is therefore probable that these varieties would afford slight differences in their chemical ingredients.

I am entirely indebted to Mr. Bright, of Ham Green, for being able to give the last mentioned locality of this rock; for upon informing him of my present pursuit, he desired me to examine the ridge to which I have just alluded, as he conceived it to be composed of a magnesian limestone.