Carnarvonshire; as, for instance, the greater part of the route by the old road from Pont Aberglasslyn to within a mile or two of Tan y Bwlch, which chiefly traverses the solid rock. The slates between the Mynfedd Inn and Machynlleth have the same dip and direction as those on the escarpment of Cader Idris. The houses of Machynlleth are all of slate. The country between Machynlleth and Aberystwith partakes of the same general character. Lofty hills, with steep and rapid slopes, and bounded by narrow vallies, frequently pretty well cultivated, the herbage covering even the summits of the hills, which are rounder than before. Woods of small oak, but of considerable extent, were frequent on the sides even when very steep. The rock rarely appears through the verdure, so that almost the only chance of gaining information respecting its nature is to be found in the little quarries beside the road, or where, as is sometimes the case, the unbroken rock constitutes the road itself. Many instances of this occurred soon after leaving Machynlleth, affording the opportunity of ascertaining that the direction of the cleavage, and the dip, still continue the same. The rock, however, is not always a pure slate, since it is very commonly interstratified, as every where else, parallel to the direction of its cleavage, with thin layers of a granular stone, in its external character greatly resembling some of the varieties every where observable in the more northern mountains, but of a colour more nearly approaching that of the slate, and often consisting of variously coloured particles lying parallel to the plane of interstratification. We regret omitting the opportunity of examining the slate quarries of Aberystwith. We walked, however, to the bold projecting rocks at the point on the NW of the town, where we found almost the only instance of irregularity in the dip, and contortion of the strata, that we have observed. At the point, and as far as we could see the coast beyond it on the north, the dip appears to be nearly E at a low angle, although that of the rocks at its foot, which are covered at high water, dip in another direction; and during the very short time we looked at these rocks, it appeared to us that those immediately in contact with the point, towards the town, and which are contorted, dip in a third direction; but it is not improbable that further investigation might have proved the inaccuracy of some of these appearances. Every where the slate is interstratified parallel to its cleavage plane with a more or less granular rock, much resembling some of the finer-grained varieties of the base of Moel Shabod. The same kind of country continued to the Devil's Bridge, and very slight opportunities of examining the rocks on one side only of the road, afforded us only the information that the summits of the hills are of slate, often in very small rhombic pieces, as though, from some cause or other, they had been shattered.. Close to the Devil's Bridge, on the side beyond the Inn, there is a quarry of slates, of which the cleavage runs as usual NE and SW, dipping towards the NW at 50°; but on the northern side the rock immediately adjoining the bridge, the dip and direction are very different. It is, however, manifest that this mass, part of which forms the abutment of the bridge, is not in situ, since its seams are opened, and the rock itself appears to have been shattered in its fall. In the deep ravines of the singular scenery opposite to the Inn, we anticipated an opportunity of ascertaining the nature of the rocks of this place. On descending these ravines to view the fine waterfalls beneath the bridge, we found the dip and direction agree uniformly with those of the quarry above-mentioned. We afterwards went to see the grounds belonging to that boast of all the tourists, Hafod, and on the banks of the Ystwith, traversing its beautiful valley (in which it appeared to us that art had done much, but nnture more), we had numerous opportunities of ascertaining that the dip and direction of the slates agree with those near the Devil's Bridge. In both places, the slates still continued to enclose layers of a granular rock resembling that so often noticed. Occasionally also, it occurs in blocks and kernels, and is so soft as to yield easily to the knife, and in several instances was observed to decompose in the same cellular manner as some of the chloritic slates of the summit of Snowdon and other places, the cells, however, being much smaller. Near the bridge at the termination of the Hafod grounds on the road to Tregarrow, were some large masses of slate enclosing layers of the rock in question, parts of which were so far decomposed as readily to break down into a perfectly soft substance of an ochreous-brown colour. Hitherto, therefore, the slates and slaty rocks appear to partake largely of the characters of those forming the more mountainous regions of the most northern parts of Wales; and it may be observed that hitherto we have not seen a single rock bearing in any degree the character of greywacke. Between Hafod and the Devil's Bridge, the country continues, as between the latter place and Hafod, bold, but extremely sterile, the lofty hills being so completely covered by coarse verdure to their summits, that the rock is visible chiefly along the water-courses in the bottoms of the valleys. Few trees, and as few attempts at cultivation, are visible on the sides of the road. Slates enclosing masses and layers of the same varieties as those prevailing between Machynlleth and the Devil's Bridge are occasionally to be observed, and possessing the same line of bearing. Just before arriving at Rhyader, however, the character of the country became changed: the hills surrounding that place are lower, and have rounder summits than any that we have previously observed; while the broader valleys offered the reverse of the picture we had so lately seen, being well cultivated and wooded. We now turn into the valley of the Wye, and 3. about a mile south of the town, an abrupt cliff of considerable height attracted observation from the nature of the rock, which is in pretty thick beds or layers, dipping to the NW about 35o, and consists apparently of minute portions of crystalline quartz firmly adhering, and presenting innumerable small ochreous specks, as though some one of its constituents had suffered decomposition, and thus constituting a paste which included somewhat round (perhaps rolled) masses of granular quartz and of hornstone (?) of considerable size, and here and there small transparent crystals of felspar. This rock altogether greatly resembles some varieties of the more compact and quartzose beds of the old red sandstone. The appearance of a rock possessing characters so greatly differing from every thing that we had seen in North Wales, indicated a complete change in the geological features of the country. This, however, did not altogether prove to be the fact, for we afterwards observed repeated instances of the same slates, and included rocks, as had been noticed before; but still it appears to us that an investigation of this part of the country, and particularly of the immediate neighbourhood of Rhyader, would prove of great interest to the geologist, as affording him the opportunity of observing rocks of very different characters in a very short compass, in such a manner, as to prove their connexion and possible transition from the one into the other. Between Rhyader and Built we also observed a rock most essentially differing from any of the preceding. It has the appearance of an indurated clay, which sometimes appears in layers, including masses of the same substance, which, by exposure, open concentrically, and finally break down into a clay; and it is only on the assumption of the prevalence of this indurated clay to a considerable extent, that we are enabled to account for the appearance of some large and high commons in this route, having pretty level surfaces. About a mile on the north of Built, we observed in the bed of the river a rock which may, perhaps, be the greenstone noted in Mr. Greenough's map as belonging to the coal formation. It is an extremely fine-grained rock, nearly black, traversed by veins of quartz, but the component materials of the rock itself are not discoverable by the help of a glass. In the bed of the river on the left of the bridge, on entering Built, we observed a shale much resembling that of the coal formation, containing large spherical masses, often in the form of septa, of a substance which is very ponderous, and considerably resembling that of the septaría enclosed in the London clay. It also contained impressions of vegetables. After leaving Built towards Brecon, the country still continues to improve infertility, and the hills are lower, but still few openings appear, and scarcely a rock is visible above the surface. Within a mile of Built, the soil begins to be tinged of a reddish colour, and about three miles from it we observed a small quarry on the top of a hill, and near the road side, situated in a sandstone perfectly resembling the old red. It afforded two or three varieties; one of them consists apparently of an indurated clay or marl of a red colour, enclosing specks of mica; another of grains of siliceous sand, and an ochreous substance connecting them; a third resembling the sandy variety, except that it was rendered slaty by the intervention of close layers of mica; these varieties are interstratified, and dip at about 15° to the SW. In conclusion, we have to observe, that previously to our quitting the hospitable roof of Mr. Dawson, at Bangor, he informed us that some varieties of the rocks of the district we visited had been pronounced by certain French geologists to be the steaschiste of Brongniart. Since committing the foregoing pages to the press, we have consulted the description by that eminent mineralogist of the Geology of the Cotentin inserted in the 35th volume of the Journal des Mines, and his particular description of the steaschiste to be found in his "Essai d'une Classification Mineralogique des Roches mélangés," in the preceding volume. The perusal of these at once convinced us of the relation existing between the rocks of North Wales and those of the Cotentin, and even of their identity in so far as related to their actually consisting of the steaschiste, and of that alone; for we did not perceive any rock whose character sufficed to raise a doubt of the whole being of one formation. We refer the reader to the two memoirs above cited, the perusal of which will readily satisfy him of the correctness of our present views of the nature of the rocks in question, and that their proper and expressive designation is steaschiste; of which we have described most of the varieties mentioned by Brongniart: they are as follow: Steaschiste rude; porphyroïde; noduleux; steatiteux; chloritique; diallagique (ours is rather augitique); ophiolin; phylladien. Having sent to Mr. G. B. Sowerby, of King-street, Coventgarden, all the impressions of shells found by us in Wales, and requested of him some remarks upon them, which his intimate acquaintance with conchology well qualifies him to afford, we annex the communication received from him on the subject, first observing that the impressions figs. 1, 2, 3, 4, 5, and 9 (Pl. XVII), are from the summit of Snowdon; 6, 7, 8, 10, 11, 12, and 15, are from the road side near Pont y Cyffin; fig. 13 is from about midway between the Devil's Bridge and Rhyader; fig. 14 from Cader Idris. |