able extent, preserve a parallelism among themselves. It must at the same time be observed, that the largest veins, which range in a direction nearly east and west, are accompanied by fissures, which divide the whole mount into parallel vertical beds. Between these beds are found oxide of tin, wolfram, apatite, the topaz, and some of the rarer minerals of the county. It would be difficult to prove that these fissures are not posterior to the consolidation of the granite. The minerals contained in them may therefore be accounted for ou the Wernerian hypothesis of infiltration.

It is not however my intention to give any account of the phenomena presented by the metalliferous series of Cornwall, or to venture on any opinion respecting their origin. In these imperfect details, I wish to confine my observations to a description of the great formations of the county. Such a description will necessarily embrace those varieties, which are strictly of contemporaneous origin, whether they present themselves in the form of veins irregular concretions, or imbedded masses. Accordingly, those mineral aggregates only, are described, which have appeared in some of the above-mentioned forms, and are supposed to be coeval with the granite. They might perhaps, without impropriety, be considered but as modifications of the granite; produced at the consolidation of the mass by the prevalence of certain ingredients in one part rather another; or by the anomalous action of crystalline forces, arising from disturbing causes with which we are unacquainted. These opinions seem to gain great confirmation, when it is recollected that perhaps all the ingredients of these anomalous rocks are occasionally found disseminated through the mass, and then appear to form a true constituent part of the granite.

By way of conclusion, it may be proper to observe, that the great extent of the formation here considered, its mineral character, the simple minerals which abound in the veins by which it is traversed, and its relation to the superincumbent rocks, all lead to the

same conclusion: viz. that it is a true granite, the oldest primitive rock of the Wernerian series.

§. 4. Junction of Killas and Granite, Granite Veins, &c.

Having in the last section enumerated some of the principal characters presented by the granite, I shall now proceed to consider the phenomena exhibited at its junction with the killas. These phenomena may be examined, with some advantage, in the beds of the many rivulets which descend from the high granitic tract, and intersect the line of junction*. Unfortunately, however, in these instances, there is never a sufficient denudation to allow of an extended examination.

In other parts of the interior, where the granite and the schist are not laid bare by such causes as we have mentioned, it is in vain that we look for their immediate contact: for the junction is almost universally obscured by alluvial matter and vegetable soil, which seem in all cases peculiarly to abound near the line of separation of two different formations. Fortunately, we are opposed by none of these difficulties in examining the western extremity of Cornwall. For the cliffs present us with a series of natural sections, in which all the phenomena we are attempting to describe are exhibited, under circumstances most favourable to investigation.

An account of such phenomena, in the order in which they present themselves through the whole line of coast, would far exceed the limits of this communication. I leave such detail with less reluctance, as several parts of the coast I am now considering will be described in the next volume of the Cornish Geological Transactions, by one whose opportunities of investigating this subject have been greatly superior to my own †. A minute account

* See the specimens taken from the bed of the river above Ivy Bridge. Devonshire series, Nos. 12....20.

+ Dr. Forbes, Secretary to the Cornish Geological Society, and Honorary Member of this Society.

will be given only of one or two portions of the cliffs which have not yet been described, and such observations will be added as are immediately suggested by the specimens placed in our cabinets.

Before we proceed to these descriptions it will not be improper to enumerate the different points on the coast where the granite and killas may be seen in contact.

1. At Trewavas Head, about two miles west of Porth Leven.

2. About half way between the last mentioned place, and the east side of Pra-Sands, where a patch of slate occupies the base of the cliffs for about 300 feet.

3. At the eastern extremity of Pra-Sands.

4. At St. Michaël's Mount.

5. At Moushole, west of the pier. Only to be seen at low water.

6. About half a mile west of Lemorna Cove.

7. About half a mile farther west near Carn Boscawen.

8. At the end of Whitesand Bay. Covered at high water.

9. Cape Cornwall.

10. Pendein Cove.

11. About a quarter of a mile west of Polmear Porth.

12. Wicka Pool. The cliffs between this and the preceding junction are formed of a band of slate, in some places, especially east of Zennor Cove, of inconsiderable thickness, and traversed by many granitic veins*. That part of the cliffs near Wicka-pool, in which the granite re-appears, is of very small extent. From the headland nearly opposite the Carracles to St. Ives' Bay, the killas rocks prevail without interruption; but the denuded surface is in one or two instances sufficiently near the central mass to be traversed by the veins of granite. With the exceptions of such portions of the coast as are contained within the limits abovementioned, the central granite does not appear in any of the cliffs of the county.

* The island immediately west of Land's-end, on which the light-house has been erected, is said to exhibit a junction of the granite and killas.

In following the line of coast, from the granite to any of the superincumbent masses of slate at the points of junction above enumerated, we often observed those various mineral aggregates, which are described in the preceding section; but we in no instance remarked such a general change in the texture of the fundamental rock, as indicated a passage towards a new formation. The line of contact is indeed sometimes exhibited under circumstances of great confusion. In no case however did there appear to be any thing like a separate formation, making a connecting link between two distinct deposits. When the division between the granite and superimposed slate was perfectly well defined, we remarked an almost instantaneous passage from one rock to the other. A very limited exception to this rule has been observed in the parish of Constantine, where a variety of granite, containing a great abundance of mica and of distinctly slaty texture, is found at the junction with the killas. (See Granite, No. 25.) This appearance is, I believe, of inconsiderable extent; for in passing through that part of the county, I was not able in any instance to find it in situ. The same observations will apply to the whole region between Dartmoor and Land's-end, which may be considered as possessing the same simplicity of structure, and almost entirely deficient in all those primitive formations which geologists have interposed between granite and clay-slate.

It is not my intention here to enter on any details respecting the killas: I may however observe that at its immediate junction with the granite, to which it generally adheres so intimately as not to be separated without considerable force, it contains an unusual proportion of mica, and sometimes exhibits that wavy texture which is so characteristic of many beds of gueiss. This variety is never of any great thickness, sometimes not more than the fraction of an inch; and then passes into a schistose rock of a purple colour; most frequently of an homogeneous texture, though sometimes exhibiting a cloudy or striped appearance, from its intimate association with a

variety of compact felspar. This rock is succeeded by and alternates with other schistose masses which will be described in their proper place.

We have before observed, that the slate reposes conformably on the granite. With reference to the general structure of the county, the assertion is undoubtedly true; in considering the structure of any particular district, it must be received with limitation. Thus at all the junctions we have enumerated, we may observe the schist dipping from the great protruding masses of granite; yet we by no means find the inclination and line of bearing of the slaty beds universally to correspond with the surface of the rock on which they rest. St. Michael's Mount affords an excellent illustration of this fact. It is almost entirely composed of granite, which on every

side presents a series of abrupt precipices.

In crossing from Merazion, the killas rocks are found suddenly to change their dip and rise towards the mount; at the base of which they may be traced through about one third of its circumference*. Though undoubtedly resting on a rock, the visible face of which is so nearly perpendicular to the horizon, few of these beds reach an inclination of more than ten or twelve degrees, and consequently appear (especially near their western termination) rather to abut against, than repose upon the granite. This circumstance, combined with the appearance of those veins which starting out from the granite traverse the superincumbent slate in all directions, has

* Near Penzance the slate rocks dip to the east, declining from the granite which appears immediately to the west: while the cliffs on the other side of the bay near PerzanUthno dip to the west, obeying a similar law with regard to the granite which breaks out farther east. Between the two last mentioned places, the beds in succession dip to all the intermediate points of the compass. With the knowledge of such facts as these, it appears extraordinary that any one should have considered the direction and inclination of the schistose rocks on the north side of St. Michael's Mount as anomalous. They are, I think, exactly what would have heen expected à priori by any one who had attended to the structure of the county. See Geol. Transact. vol. I. p. 144.

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