Transactions of the Geological Society, 1st series, vol. 4/On a Whin Dyke
By the Hon. HENRY GREY BENNET, M.P. F.R.S
vice-president of the geological society.[Read 6th March, 1812.]
I BEG leave to communicate to the Geological Society some particulars of a whin dyke, which traverses a portion of the limestone strata in the northern district of the county of Northumberland, and projects into the sea on its north-eastern coast.
It is seen most distinctly at Beadnel Bay, and may be traced a few miles inland from this spot, where it forms a species of pier into the sea. The surface strata have been washed away, and the dyke itself is left some few feet higher than the rock on each side of it. Upwards of 300 yards are thus seen at low water running in a right line from the sea towards the north-west, 27 feet in width. It rises in a perpendicular position through all the strata, without making the least alteration in the dip or inclination of those that are adjacent: but some short distance from the place where it is laid open to view, the limestone strata are much broken and dislocated. The qualities of the different strata in contact with the dyke differ materially from those of the same strata at some distance from it, particularly the limestone, which when lying in the immediate vicinity of the whin will not burn into lime of any value. This deterioration diminishes in degree as the distance from the dyke increases, and it is about 20 feet before the limestone acquires its perfect properties of burning into good lime. The same thing is observed in both the strata of limestone. The stratum of what I shall term a species of tuf, composed of felspar and carbonate of lime, is indurated as it approaches near to the whin, and it then resembles in structure and colour the whin itself; it is much fuller of joints near the dyke, than at a distance from it. In no case did there appear to be a complete junction of the whin with the limestone, or with the tuf; but there is invariably a small fissure, that seems to separate them to a great depth, on the edges of which the limestone sometimes assumes a sparry structure, and is in some places considerably mixed with pyrites. In one part of the dyke a piece of the tuf is enveloped in the middle of the whin; this fragment is about 60 feet in length, and 2 feet in breadth at its broadest part, and the two ends terminate in points.
The following is a section of the strata, obtained from the information of the proprietor of the lime works.
|Black metal, argillaceous slate||27|
|Slaty and micaceous sandstone and black metal Coal, a thin seam.||27|
That part of the strata, which the workmen called argillaceous, I had not an opportunity of examining, at least where it was in contact with the whin, it being under water. Neither could the coal be seen, as it is at too great a depth.
The strata dip about one yard in six to the south; but their general inclination in this district is to the east and north-east.
The specimens that accompany this paper will shew to the common observer the difference between the limestone when in contact with, or at a distance from the dyke, as well as that between the dyke itself and the ordinary whin rocks of the county, such as those which are found on the Cheviot, and which form large masses on that range. Indeed all the whin dykes that I have seen in the northern district of Northumberland, the two which are so near to each other in Holy Island, and those which form the Fairn Islands, no less than that at Beadnel, bear a striking and uniform resemblance to each other; and are unlike those ranges of whin which are composed principally of hornblende, and which prevail to such an extent in the north-western parts of the county.