this district is not quite vertical, and the direction of the differently-coloured layers not quite horizontal; but they are never at right angles to each other; for it uniformly appears to be the case, that if it were possible to divide the latter along the lines separating the differently-coloured parts from each other, and all along the cleavage in the direction of the slate, we should reduce them into rhombic masses, agreeing in form with those often observable in slates, owing in the one case to a species of natural cleavage, or in the other to the progress of decompo

sition.

A consideration of the nature of these specimens just described, and more especially the inspection of the masses exhibiting the same effects on the large scale, amply account in our estimation for the appearances of stratification on the summit of the ridge connecting the Lluwydd with the Widdfa. It would account at least for the resemblance of a nearly horizontal stratification; and from examination of the spot, we are inclined to believe that the appearance of the dip and rise of the seeming strata is to be attributed only to the actual inequalities of its surfaces. A basin offering the same appearances of stratification occurs at the head of Llyn Idwell, in a branch of the Glyder Mountain, which we could not visit, but have no doubt they may be attributed to the same cause.

The continuation of our walk beneath the summit of the Lluwydd for about a mile, and a quick descent at its termination into Cwm Llan, offered no change in the nature of the rocks. The upper part of the descent of the Cwm is composed of blue slates, but near the bottom appears a long and thick ridge in the direction of NE and SW of the rocks so often noticed as occur ring abundantly at the base of Moel Shabod; these continue to the foot. The dip of the slaty cleavage on the whole of the descent is towards the NW at about the usual angle of 54°. The appearance of roundness on the summit of the rocks heretofore noticed is remarkable in this neighbourhood.

Not far beneath the summit of the Lluwydd, which is 3000 feet above the sea, is a copper mine, the principal vein, according to the information of an intelligent miner, runs about NE and SW, and is about 18 inches wide. It possesses several strings or leaders, on some of which they were working; these did not appear to have any regular walls, and seemed to run nearly N and S, and to consist of a multitude of small strings traversing a rock greatly resembling that already described as being chiefly visible among the rubbish of the Widdfa mine, and, like it, containing minute crystals of iron pyrites.

In passing along the road from Capel Curig to Beddgelert, we perceived no rock with which we were not already acquainted; and for about two miles north of Beddgelert towards Carnarvon, only the same varieties occurred. The same obser

6

vation applies to rocks of the Pass of Pont Aberglaslyn, where the same rocks and slates are manifestly interstratified. Just beyond Pont Aberglaslyn, the road to Tan y Bwlch divides; the new road being more even than the old one, which traverses the elevated and barren mountain region between the two places, we walked over the latter as probably affording the best opportunity of observing the nature of the rocks. For several miles we found nothing different from those every where observed, except that, generally speaking, they are harder, and the slates more generally incline to blue. In one instance, we perceived a rock perfectly resembling those of the ruin covering the side of Penmanmawr. A short distance before we began to descend towards Tan y Bwlch, however, a change was perceived; ridges of rock in which no steatite was observed were interstratified with the slates parallel to their cleavage plane, and these running in the direction of NE and SW, and apparently consisting only of fine grained chlorite, calcareous spar, and quartz, the latter prevailing; one ridge appeared to consist of granular quartz including a very few specks of chlorite. These rocks rose in ridges considerably above the slates bounding them on each side, and with them dipping towards the NW. at the usual angle of 54°.

Slates prevail on both sides of the vale of Festeniog below Tan y Bwlch Inn towards the sea, the flat base of the vale consisting apparently of alluvial matter: the beauty of this vale, however, which is justly celebrated, appears to be owing chiefly to the decomposition apparent in the slates wherever they are visible, forming a soil in which the woods of its northern bank flourish luxuriantly. A part of the hill on the opposite side is clothed in like manner, but the trees are of less height, the soil in which they grow being also a slaty rock in part decomposed, or in fragments generally so very small that we could not perceive the direction of the cleavage plane. On the summit of the hill, however, just before we began the descent to a waterfall termed the Rhyader Dhû, about two miles from Tan y Bwlch, the rock did not differ from those of the base of Moel Shabod, and is interstratified with slates: a fallen mass appeared in the descent to the waterfall consisting of the same rock enclosing round or ovate masses of a quartzose substance. In the basin of the upper waterfall, we observed a new appearance in the arrangement of the rocks so often noticed. The slates here appeared in nearly a horizontal position, and exhibited a tortuous course,. which we have never before observed, while the rocks lying between slates assumed a somewhat columnar form, thus:

We afterwards traversed in our way to Harlech, a country consisting of the usual rocks and slates, possessing but little vegetation for some miles. Quartz often prevailed so greatly in the rock as to give it a character approaching to that of quartz rock. The precipice overhanging Llyn Tegwin, which is very lofty and unusually rugged, consists chiefly of slates presenting a rhombic form, probably from weathering, the acute angles of the monstrous rhombs protruding from the face of the precipice. Two layers of considerable thickness of the usual rock were interstratified parallel to the cleavage plane of the slates, the one near the summit, the other at the base; they appeared parallel, the direction of the latter being towards the north-west at an angle of 30°, being a much lower angle than we had before

seen.

At about the distance of four miles from Harlech, and while traversing a valley chiefly, if not altogether, of slates more than usually brittle, and apparently in a state of decomposition, and admitting of the growth of considerable woods of small oaks, we perceived on our right hand two lofty hills, each presenting to us a rugged scarp, consisting of slates, having precisely the same appearance as those overhanging Llyn Tegwin, and crowned, one of them in a remarkable manner, by rocks in the form of closely aggregated columns, of the height, as well as we could judge from below, of 30 to 40 feet. These rocks dip to the NW, and though we had no opportunity of inspecting them, we inferred from what has been described as occurring in the ample basin of Rhydr Dhû, that they did not differ from the usual rocks of the district.

The long ridge, at the southern termination of which Harlech Castle stands, and which presents its side towards the sea, consists, in its upper beds, of a coarse slaty rock manifestly appertaining to the rock so constantly observed, of which its lower beds consisted; quartz, however, prevailing in it in a more than common degree.

In the first three or four miles from Harlech towards Barmouth, a rock in situ is scarcely perceptible near the high road, the country being considerably flat, and affording a scarcely interrupted covering of herbage. If, however, we may be allowed to judge in some degree of the nature of a country by the rocks composing its stone fences, which in Wales present almost every variety that is to be seen in situ, it may be concluded that the same rocks still prevail. The remaining part of the way, there was not sufficient light for us to observe any thing correctly. The road, however, is by far more rugged than we had found road in any of North Wales. The cause of other part this was perceived the next morning on retracing our steps about a mile, in which the granular rock prevails in most, if not all, the varieties observed at the base of Moel Shabod. In some places, however, quartz entered into its composition in greater abundance than in any rock perceived by us on that mountain.

any

On quitting Barmouth for Dolgelly, we found that the very last house stands on the termination of the granular rock, and that the slates perceptible on the hill behind the town descend to this place, and repose on the rock in a line perfectly consonant with the plane of the slaty cleavage, which as usual runs NE and SW, and dips towards the SE at an angle of 68°. The slate here is somewhat coarse, though with a completely slaty structure it is translucent on the thin edges, and of a green colour by transmitted light.

Slates continue to prevail along the road from Barmouth to Dolgelly, and they are in many places newly laid open by the recent widening of the road, and for materials for its repair. The direction of the lamina continues, as we observed it immediately on quitting Barmouth, the whole of the way with the same dip, the angle somewhat declining in approaching Dolgelly.

But though slates prevail in the mountains on our left from the base to the summit, we were able, in several instances, to perceive, before our arrival at it, a change in the nature of the rock, by the appearance up the side of the mountain of a ridge differing manifestly from the slates, and which, on examination, proved to be a rock of the same nature as the base rock of Moel Shabod, but often without any appearance of quartz or chlorite, and interstratified with the slates in the direction of NE and SW. The rock of the only ridge which we carefully examined, appears to consist of steatite occasionally mixed with carbonate of lime, since it effervesces in patches. It is fine-grained, yields readily to the knife, of a greyish colour, and translucent on the edges.

In approaching Dolgelly, we had a good view of the summit of Cader Idris, and of some ranges of lower mountains, which may be considered as forming a part of its northern side. These possess a character different to that of the summit of the mounNew Series, VOL. IV. 2 E

tain in being much whiter, and more rugged; while in the distance, the line of their extended summits is more nearly horizontal than those of any other mountains we had seen; and there are still lower ranges, apparently consisting of the same rock, running parallel with them towards the sea coast. It is undoubtedly difficult to determine the precise run of these ranges in the circumstances under which we viewed them, but we judged by the bearing of the compass, that the run of their ridges is in the direction nearly of NE and SW; that is, parallel to the direction of the cleavage plane of the slates here and almost every where else.

In our route to Machynlleth, we crossed Cader Idris, descending by Craig y Caie to the Mynfedd Inn; and being aware that it is the intention of Mr. Aikin, ere long, to present to the public a detailed account of the geology of this mountain, we abstain from more than a general remark or two, viz. that many varieties of the rocks prevalent at Moel Shabod appear likewise in this mountain, the escarpment of which towards the north consists of columnar rocks, bearing generally more completely the character of greenstone than any rock we have observed on the north of that mountain; that on the face of this escarpment are visible two or more beds of slates, interstratified with the columns of greenstone, the slates' resting on the summits of the lower columns, while the bases of the upper rest upon the slates; that the plane of the slaty cleavage is about at right angles to the position of the columns, running NE and SW, and dipping at about the angle of 68° to the SE.

In ascending the mountain, we found several impressions of a shell differing from those observed on the summit of Snowdon, and also from those found near the base of Ben Glog and at Moel Shabod, in a rock which is fine-grained, soft, apparently composed chiefly of steatite, and perfectly resembling a variety occurring at the base of the latter mountain, and in many other places.

The road to Machynlleth runs chiefly along narrow vallies often bounded by lofty and steep hills. For the first two or three miles, their sides, though verdant to the very summits, are neither well wooded nor cultivated, and wherever a rock became visible, it was always slate, nor did any solid rock appear during the whole of the route to Machynlleth, either by the sides of the road, or in the form of ridges as heretofore; the stone fences are of slate, and, the road being composed of the same material, is, though hilly, superior to most we had lately travelled. After the first two or three miles from the Mynfedd Inn, extensive woods of young oaks covered the sides of the mountains often nearly or quite to their summits, almost the whole of the way to Machynlleth; while here and there the oak, the ash, and the sycamore, were of a considerable size; the appearance on either hand forming a perfect contrast to the general scenery of