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REMARKS.

Eighth Month.-1-5. Fine. 6, 7, 8. Cloudy and fine. 9, 10, 11. Fine. 12. Cloudy morning: fine afternoon. 13. Drizzling morning: very fine afternoon. 14-23. Fine. 24. Fine day night rainy. 25. Fine. 26. Showers. 27. A thunder storm about noon: showery. 28. Showery. 29. Fine. 30. Fine: showers in the evening. . 31. Fine.

RESULTS.

Winds: N, 1; NE, 1; E, 4; SE, 1; SW, 4; W, 9; NW, 11.

Barometer: Mean height

For the month...

For the lunar period, ending the 10th.......

29.773 inches.

29.846

For 14 days, ending the 5th (moon south)........... 29-818

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ANNALS

OF

PHILOSOPHY.

NOVEMBER, 1822.

ARTICLE I.

Sketch of the Geology of Snowdon, and the surrounding Country. By W. Phillips, FLS. MGS. and S. Woods, MGS.

THE structure of North Wales, as far as our knowledge extends, has hitherto but in a slight degree excited the curiosity, or exercised the judgment of any person conversant with geological inquiries. It has been assumed, however, that in North Wales the vallies are occupied by clay-slate, and the mountains or their summits by greenstone. The discovery of organic impressions near the summit of Snowdon created doubts which were not easily solved; and while some affected to dispute the accuracy of the observers, others accounted for it by gratuitous suppositions. We believe that no part of this country can be called primitive, and that neither greenstone, clay-slate, nor greywacké, is any where to be found in the district of Snowdonia: in this term we comprehend the mountain range bounded on the east by the Vale of Conwy, extending a line southward to Festiniog on the north by the Bay of Beaumaris: on the west by the Menai Strait carrying on a line from Carnarvon to Pwlhellie, and on the south, from thence along the coast of Cardigan Bay, through the Vale of Festiniog.

In our opinion this district offers a new and highly interesting field of investigation to the geologist. The partial examination of its various rocks, and especially of their extremes, would inevitably lead to the conclusion that they are of different formations, and possess distinct characters: a more extensive survey affords satisfactory evidence that most, if not all, of them, graNew Series, VOL. IV.

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dually and imperceptibly pass into each other; that hence they maintain no uniform or characteristic difference, and that all their numerous varieties are interstratified with slates composed of the same materials, and consequently that they are all associated in close family alliance, and owe their existence to a common origin. There are no indications of any one of these slates or rocks being superior or inferior to the rest, and although some of these rocks might be pronounced to be greenstone by a casual observation of them, yet the occurrence of the same organic impressions near the summit of Snowdon and in several other places clearly indicate them to be members of one and the same series. The greater proportion of these rocks may be found within a short compass from Capel Curig, and the situations in which from our own limited experience we should recommend as the most instructive to observers are, the section of the road near Pont y Cyffin; both the new and old road towards Bangor, and beyond their junction from the excavated pass at Ben Glog to a mile or two beyond the inn at Tan y Maes towards Bangor; but above all the mountain close behind the inn called Moel Shabod from the base to the summit on every side. We had to regret that a continuance of unfavourable weather obstructed our visits to the Glyder and the Trefan, but from the information of our intelligent friend Mr. Dawson, of Bangor, who has accurately surveyed the whole country, and a comparison of his specimens, we have every reason to believe they would have furnished similar results. We pretend only to give a hasty and imperfect sketch; limited time and abundant rain prevented us from attempting more, and we proceed to state in detail the evidence we have procured in the hope of stimulating the lovers of the science to investigations still more minute and satisfactory, which will in that delightful region carry with them their own reward.

The general character of the rocks of Snowdonia, and which extend even into South Wales, is of a nature that we did not at all anticipate. Such as possess little or no appearance of a slaty texture, and these are often porphyritic, have for their base a substance greatly resembling steatite or potstone,* often so soft

* Some of the rocks of which this substance forms the base, or which are constituted of it, possess so nearly the characters of some varieties of steatite or potstone, as to prevent all hesitation in pronouncing them to be allied to those substances, while others assume a talcose character; their connexion with chlorite (which almost always accompanies them) seems to authorize the conclusion that they are all of one family, since the greater number of these minerals analyzed by Klaproth, Vauquelin, and Lampadius, were found to consist of the same elements; namely, silex, alumine, with 20 to 30 per cent. of magnesia, and a small proportion of lime. Hitherto, however, we have been speaking of the resemblance of the rocks of Wales to steatite or potstone, only from their external characters, and judging by these alone, we repeat that no hesitation would be felt in considering them as varieties of those substances. Being anxious, however, to ascertain by a reference to chemical agency, how far the chemical characters agree with the external, we placed in the hands of Mr. Richard Phillips five specimens, vary ing considerably in aspect, but all allied by interstratification. These were found to

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as to yield readily to the knife, sometimes even to the nail, but occasionally so intermixed with siliceous matter, as scarcely to receive any impression from the knife: in the latter case it is generally of a greyish colour; in the former greenish, the colouring matter being in our estimation chlorite, which often is so arranged as to impart to the rock a slaty structure; and we possess the most unquestionable proofs that the impressions of shells occurring within 10 feet of the very summit of Snowdon are in a rock of this nature, of which doubtless the occasional fineness of the grain, and its generally slaty structure, has given rise to the notion that these impressions occur in a greywacké slate. The imbedded substances, when the rock possesses a porphyritic aspect, appear to be contemporaneous nodules harder than the rock itself, quartz, more rarely felspar, frequently carbonate of lime, the three latter being generally crystalline, transparent, and very minute; chlorite, however, is so commonly an ingredient of all the steatitic rocks, either in very minute particles, or in layers, that it may be said to form almost an essential ingredient, and is commonly present in so large a proportion as to impart its greenish colour to the mass.

Chlorite, however, sometimes prevails so greatly, as almost to exclude the other ingredients, and then appears in the form of chlorite slate, which occurs near the summit of Snowdon, within perhaps 20 feet of the steatitic rock containing the impressions of shells; and is even interstratified with layers of the same nature. The two extremes, therefore, appear to us to be steatite and chlorite; but each of these two substances is often so modified by combining with the other in different proportions, and probably also by the intimate dispersion through the mass of siliceous matter, and by its occasionally imbedding small nodules of calcareous spar, felspar, and grains of quartz (the latter in one instance prevailing to the almost total exclusion of all the rest), that the rocks assume a great variety of aspect, and even appear to differ so greatly, that nothing short of an inspection of the whole series, or seeing them either interstratified or passing into each other, as we have mostly seen them, would suffice to produce a conviction of their actual and even intimate connexion.

The slates forming so considerable a proportion of the sur face of this country, are also of very different aspects; varying from nearly pure steatite of a greenish colour, soft enough to yield to the pressure of the nail, through still harder varieties, both externally and by transmitted light of the same colour, to

consist chiefly of silex and alumine, but included a small proportion of lime. On a repetition of his experiments for the purpose of ascertaining whether an alkali is or is not present in a rock most nearly resembling steatite, no alkali was detected, but the presence of a very minute trace of magnesia was indicated. These rocks, therefore, differ from steatite and potstone in containing little or no magnesia; but as it is essential to adopt some name for the sake of reference in the following pages, we shall in speaking of them use the term steatite, which may serve until some more appropriate designation shall be given to them.

the blue and purple varieties of ordinary slate. These all occur interstratified with the rocks above-mentioned, and certainly partake of their nature. The slates of the vast quarries at Nantfrancon dip beneath these rocks towards the south east, while similar rocks occur at the coast on the north-west of Bangor: on these rocks we incline to believe the slates of Nant-francon actually rest; but having had no opportunity of ascertaining the fact, we recommend its investigation to future observers. Of this, however, we are assured, that we have often perceived similar slates interstratified with the same rocks, and conclude from this and other circumstances, that the slates of that quarry strongly partake of the nature of chlorite slate.

One fact, and we consider it as a somewhat remarkable one, is, that the plane of the cleavage of the slates and slaty rocks runs everywhere (with the exception of one hill) from the east of north to the west of south, the slates being in some few instances vertical, but more commonly dipping at a high angle towards the west of north, or east of south. From this circumstance, it may, perhaps, be argued with much probability, that even the most mountainous tracts of North Wales do not present any appearance of that kind of disturbance, which, in some countries, is not uncommon, and which is considered to have arisen from depression on the one hand, or elevation on the other. The slaty cleavage being, as we have already described it, it follows of course that no mantle-shaped masses were observed.

We are not disposed to view the circumstance of the almost uniform direction of the plane of the slaty cleavage as an isolated fact, relating simply to the geological structure of Wales, but as being probably connected with that of our island generally; for it is well known that the newer strata of England possess the same general bearing, as may be perceived at once by casting the eye over a geological map of our country. When thus viewed in connexion with a series of numerous beds, this point appears to assume an interest at once both important and extensive.

Shells occur in greater abundance, and of more varieties, than we expected to find.

In North Wales, we did not perceive a single instance of contortion either in its rocks or slates.

With the intention of ascending Snowdon by all the customary routes, our first station was at Capel Curig Inn, but for several days our hopes of making an ascent were disappointed by the wetness and haziness of the weather, which prevented us from even discerning the mountain, and ultimately deprived us of the opportunity of ascending it from all the various points. Meantime we were induced by Mr. Dawson, who did us the favour of a visit at Capel Curig, to ascend and examine the neighbouring mountain Moel Shabod. Previously to our ascent, the gentleman above named mentioned to us one circumstance which for

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