Among the specimens in the Woodwardian Museum, will be found,

Nos. 18-21; belonging to the first variety; from an Elvan about ten feet thick near Pentowan. It ranges nearly east and west, underlies to the north at an angle of about 45°, and appears imbedded in the slate. The beds in contact with it are undisturbed and unaltered.

Nos. 3, 4. From a very beautiful Elvan, more than 40 feet wide, which appears below the high-water mark, at the western extremity of Pra-Sands, near the perpendicular cliffs which form Pedn-dupoint. It ranges nearly north-west and south-east, and seems conformable to the killas beds, which are neither disturbed in their position, nor altered in their texture at the junction. The exterior parts of this Elvan pass into the third variety, but the central mass is a porphyritic granite.

No. 5. From an Elvan about forty feet wide, between Camborne and the Dolcouth mine, which ranges nearly east and west, and underlies south. It is of the fourth variety, but not so striking as the preceding.

No. 6. From a very narrow dyke east of Cligga Point. It belongs to the sixth variety.

These Elvan courses are no where seen to more advantage than in the cliffs near St. Agnest. Some of them are repeatedly cut off by the extraordinary indentations of the coast, and re-appear in the successive headlands, in the line of their first direction.

* It has lately been extensively used in building the pier at the village of Pentowan. Many of the houses in Truro are built of a stone of nearly similar texture, obtained from a large Elvan near that place. See Nos. 15-17. The hewn surfaces of the blocks, quarried from the Elvans, have in many parts of Cornwall, a near resemblance to the building stone derived from the Oolite beds in the midland counties.

+ See a paper on this subject by the Rev. J. J. Conybeare, Geol. Transact. vol. IV. p. 401.

R

It is not my intention to attempt any description of the phenomena which are exhibited at the intersection of the rocks we are now describing with the metalliferous lodes and cross courses. The subject is one of great interest to the practical miner; and will, no doubt, receive every illustration from the labours of the Geological Society of Cornwall. It may however be further observed,

1. That the schistose rocks, even when most intimately associated with Elvan courses, do in no instance appear so changed, in texture and position, as to give the least support to the hypothesis ; that these porphyritic masses have been injected into the killas, posterior to its consolidation, by the powerful action of subterranean force.

2. That at the time of the formation of those fissures, which are of such constant occurrence among the metalliferous veins, and which in such numberless instances have broken the line of continuity of the great mineral beds, the Elvan courses appear to have borne exactly the same relation to the killas which they do at present.

3. That in some parts of Cornwall, which are remote from the granite, they are very rare; and at still greater distances, where the rocks are decidedly of a more mechanical origin, they disappear altogether.

The inevitable conclusion seems to be, that the Elvan courses are contemporaneous with the rocks with which they are associated. We were not able to learn whether, in any instance, the Elvans had been traced into the granite. In the very extended surface of granite, denuded in the western cliffs, we saw no veins exactly corresponding to the Elvan dykes here described. Combining this fact with the appearance presented by the lower termination of granite veins, we may venture to conjecture, that the Elvan courses may, in some instances, be traced down to the granite, but that they are in no instance afterwards continued into its mass.

When the successive beds of killas were deposited, their materials were in that state of solution which admitted, under favourable circumstances, of a development of the powers of crystallization. The resulting phenomena would depend on the actual materials presented by the solvent, whatever might have been its nature, and the greater or less prevalence of disturbing forces. If we admit this principle to account for the appearance of those highly crystalline granite rocks which are associated with the more earthy beds of schist, we are under no necessity to limit its powers of operation. The extent of these powers can only be estimated by an attention to the effects produced. On this account we venture to class the anomalous rocks of Cligga Point with the Elvan dykes of the adjoining cliffs, and to consider the whole promontory as a mere local formation, resulting from the accidental combination of circumstances favourable to the production of such crystalline masses as are above described.

To the north-east of St. Agnes, the coast is singularly broken and diversified : it is also peculiarly deserving of examination from the various stages of decomposition exhibited in the prevailing rocks, and from the mineral deposits with which they abound. About three miles from the last-mentioned place, the cliffs suddenly start off from their mean bearing, and form a bold headland, called Cligga Point, composed of granitic beds, which appear to repose on the killas, and at the junction are nearly parallel to its great line of cleavage. At low-water, one may descend to the base of the cliffs and examine these beds in detail. They are generally found to differ from the central granite, almost as much in mineral character as they do in geological position. The extreme point of the headland, which is several hundred yards from the termination of the killas, very nearly resembles the common granite of the county but the intermediate parts, which are divided by innumerable parallel fissures, exhibit such varied modes of aggregation,

and are frequently in such a state of disintegration, that it is difficult to convey a correct idea of them by description*. They are chiefly composed of quartz, sometimes distinctly crystallized, felspar, mica, and chlorite, in every variety of combination; and they are often porphyritic. We remarked one unusual porphyritic mass, composed of crystals of felspar imbedded in earthly chlorite.

The parallel fissures are metalliferous; and the operations of mining have been conducted in them to such an extent, that some parts of the rock are as open as a honeycomb. These veins abound in the ores of tin, and present some traces of the ores of copper. Wolfram, and some other metalliferous minerals, are also very generally found in them. In one portion of the headland, the parallel beds are slightly deflected, and soon afterwards succeeded by a mass so violently contorted, that all its subordinate beds are convex to the horizon. Even this part of the cliff is metalliferous ; and the veins, if I have not been misinformed, exhibit the same curvature as the beds in which they have been opened; a circumstance probably unexampled among all the other mineral deposits of the county.

The killas, at its immediate junction with the granite beds is unusually soft and micaceous; and in that state is continued till it is cut off by an Elvan dyke, not less than forty feet wide, agreeing in mineral character, as well as in the inclination and direction of its mass, with many of the beds which form part of the promontory above described. One fact, first pointed out to me by Mr. Gilby, is I think worth remarking. The separation of the granitic mass of Cligga Point from the killas, is neither parallel to the fissures in the one nor to the great cleavage of the other; but may be represented by a very irregular superficies which is, on the whole, nearly perpendicular to the horizon. In this respect, therefore, the junction of the two formations presents an analogy to the phenomena we have before insisted on.

* See the Specimens from Cligga Point, Nos. 7—14.

§. 6. On the Killas.

The wide extent of this formation and the relation of its subordinate beds to the central ridge, have been already described with sufficient minuteness. Some general notion may be formed of the mineral character of these beds from the specimens now placed in our Museum, which were selected for the purpose of general illustration. In describing the phenomena in the western parts of Cornwall we mentioned the micaceous texture of the rock in contact with the granite, and its almost immediate passage into a nearly homogeneous slaty mass, generally of a dark purple colour*. Under similar circumstances we remarked slaty beds of exactly the same appearance on the sides of Dartmoor, the right bank of the Tamer, and some other parts of Cornwall. The more compact homogeneous varieties, when breathed upon, give out a strong argillaceous odour. Their mean specific gravity is about 2. 67. When small splinters are examined with a lens, they are found to have the texture, and greasy-looking surface, of some varieties of felspar. All the specimens exhibit, in the flame of the blowpipe, nearly the same phenomena, and melt into a light transparent glass with some dark stains, seemingly derived from impurities.

After examining the beds of slate as they succeeded in order of superposition, we were not surprised to find that corresponding parts of the formation, in different places, exhibited very different appearances. During a hasty passage over the north side of Dartmoor we remarked several extensive beds of hornblende-rock and greenstone

* At the junction of the granite and slate in the Dolcoath mine we found a porphyritic rock (see Killas, Nos. 101, 102.) which differed from any bed we remember to have seen in a similar situation. In some other instances, in which we descended the mines to examine the contact of the granite and the slate, we were disappointed in our hopes of gaining any general information. The rocks in those parts where they were intersected by the veins, were either in so ruinous a state, or so much mixed with the sparry matter of the vein itself, as to assume an almost entirely new character.