We were no less gratified in our views with respect to the other object, the mineral operations by which soft strata, regularly formed in horizontal planes at the bottom of the sea, had been hardened and displaced. Fig. 4. represents one of those examples; it was drawn by Sir James Hall from a perfect section in the perpendicular cliff at Lumesden burn. Here is not only a fine example of the bendings of the strata, but also of a horizontal shift or hitch of those erected strata.
St Abb's Head is a promontory which, at a distance, one would naturally conclude to be composed of the schisti, as is all the shore to that place; but, as we approached it, there was some difference to be perceived in the external appearance, it having a more rounded and irregular aspect. Accordingly, upon our arrival, we found this head-land composed of a different substance. It is a great mass of red whin-stone, of a very irregular structure and composition. Some of it is full of small pebbles of calcareous spar, surrounded with a coat of a coloured substance, different both from the whin-stone ground and the inclosed pebble. Here ended our expedition by water.
Having thus found the junction of the sand-stone with the schistus or alpine strata to run in a line directed from Fast Castle to Oldhamstocks, or the heads of Dunglass burn, we set out to trace this burn, not only with a view to observe the junction, if it should there appear, but particularly to discover the source of many blocks of whin-stone, of all sizes, with which the bed of this burn abounds.
The sand-stone and coal strata, which are nearly horizontal at the mouth of this burn, or on the coast, become inclined as we go up the course of the rivulet; and of this we have fine sections in the bank. The Dean of Dunglass is formed of precipitous and perpendicular rocks, through which the running water has worn its way more than a hundred feet deep; above this Dean the banks are steep and very high, but covered with soil, which here is a deep gravel. The burn runs all the way up to Oldhamstocks upon the sand-stone strata; but there, these are traversed by a high whin-stone dyke, which crosses the burn obliquely, as we found it on both banks though not in the bed of the burn; it is in the south bank below the village, and on the north above it. Here is the source of the whin-stone which we were looking for; it is the common blue basaltes, of the same nature with the Giant's Causeway, but with no regular columnar appearance.
Above Oldhamstocks we again found the sand-stone in the bank, but it soon disappeared under a deep cover of gravel, and the burn then divided into several rivulets which come from the hills. We traced the one which led most directly up to the mountains, in expectation of meeting with the schistus, at least, if not the junction of it with the sandstone. But in this we were disappointed. We did not however lose our labour; for, though the junction which we pursued be not here visible, we met with what made it sufficiently evident, and was at the same time an object far more interesting in our eyes.
I have already quoted Mr Voigt's description of the sol mort rouge; he says, that in places it forms entire mountains; here we have a perfect example of the same thing; and the moment we saw it, we said, here is the sol mort rouge. We ascended to the top of the mountain through a gully of solid pudding-stone going into decay, and furnishing the country below with that great covering of gravel, soil, and water worn stones. We were now well acquainted with the pudding-stone, which is interposed between the horizontal and alpine strata; but from what we had seen to the eastward, we never should have dreamed of meeting with what we now perceived. What we had hitherto seen of this pudding-stone was but a few fragments of the schistus in the lower beds of sand-stone; here a mountain of water-worn schisti, imbedded in a red earth and consolidated, presented itself to our view. It was evident that the schisti mountains, from whence those fragments had come, had been prior to this secondary mass; but here is a secondary mountain equal in height to the primary, or schisti mountains, at the basis of which we had seen the strata superinduced on the shore. Still, however, every thing here is formed upon the same principle, and nothing here is altered except the scale on which the operation had been performed.
Upon the coast, we have but a specimen of the pudding-stone; most of the fragments had their angles entire; and few of them are rounded by attrition. Here, on the contrary, the mountain is one pudding-stone; and most of the fragments are stones much rounded by attrition. But the difference is only in degree, and not in kind; the stones are the same, and the nature of the composition similar. Had we seen the mass of which this mountain is only a relict, (having been degraded by the hands of time), we should have found this pudding-stone at the bottom of our sand-stone strata; could we have penetrated below this mass of pudding-stone, we should have found our schistus which we left on the shore at St. Helens and in the Tour burn. In Tiviotdale the vertical schisti are covered with a bed of pudding-stone, the gravel of which had been much worn by attrition, but the thickness of that bed is small; here again the wearing operation has been great, and the quantity of those materials even more than in proportion to those operations. We returned perfectly satisfied; and Sir James Hall is to pursue this subject farther when he shall be in those mountains shooting muir game.
We had now only one object more to pursue; this was to examine the south side of those mountains of Lammermuir upon the sea shore, in order to see the junction of the primary schistus with the coal strata of Berwickshire. Mr Hall was to meet us at the Press, and we were afterwards to go with him to Whitehall. We met accordingly; but the weather was rainy; and we went directly to Whitehall. I had often seen the pudding-stone in great masse; in the banks of the Whiteader, as it comes out of the mountains, but then I had not seen its connection neither, on the one hand, with the schisti, nor, on the other, with the sand-stone strata. We knew that at Lammerton upon the sea coast there was coal, and consequently the sand-stone strata; and reasoning upon those data we were sure that our proper course of investigation was to trace the river Ey to the shore, and then go south the coast in search of the junction of the schistus with the horizontal strata. This we executed as well as the weather would permit; but had it to regret, that the rainy season was not so favourable for our views, as it was agreeable to the country which had been suffering with the drought.
It is needless now to enlarge upon this subject. I shall only mention that we found the red marly strata above the pudding-stone in the bed of the Ey and its branches; we then traced the schistus down the Ey, and found a mass of the most consolidated pudding-stone upon the coast to the north of the harbour of Eymouth. But this mass did not rest on the schistus; it is immediately upon a mass of whin-stone; and the schistus is in the harbour, so that this whin-stone mass seems to be here interposed between the pudding-stone and schistus. We then pursued the coast southwards until we found the junction of the schistus and sand-stone strata about two miles from Eymouth; but here the junction was not attended with any pudding-stone that we could perceive.
Having found the same or similar appearances from the one end to the other, and on both sides of that range of mountains which run from sea to sea in the south of Scotland, we may now extend our view of this mineral operation in comprehending every thing of the same kind which we meet with in our island or any other distant country.