case, even where the greatest possible care had been taken in the selection of the materials and construction of the walls.

Under the thin vegetable soil of the moors, we may frequently find earthy siliceous beds, obviously derived from the granite; which may be considered as an exhibition of that rock in its last stage of decomposition.

It must however be observed that the rock in some instances presents an appearance entirely different from what we have described which may be considered as a decomposition in the mass. An imperfect instance of what is here meant, may be seen in a cave near St. Levan. The most instructive example of this decomposition may be examined in detail on St. Stephen's Moor. For some reason, connected probably with the chemical composition of the rock; but which analysis has, I believe, not yet been called in to explain; the greater part of that beautiful variety of granite which contains so much silvery mica, exhibits the peculiar decomposition of which we are speaking. When this rock prevails, the surface of the ground is uneven, but commonly does not present any of those scattered bowlders which I have before described. Numerous excavations are made in St. Stephen's Moor, exposing to the view a granite of a brilliant white colour intersected by conOn a temporaneous veins of quartz. near examination it is frequently found soft enough to be cut with a spade, and is in that state packed up and exported to our potteries; those parts only being rejected which are contaminated by the presence of dark coloured mica*. In other pits on the same moor, the rock is broken down by mechanical force, and a running of water is made to pass through the fragments. The finely attenuated particles of felspar are instantly taken up and carried off in streams, as white as milk,

* In some instances we found it impossible to reduce the fine powder, derived from this decomposing felspar, to a glass bead. This seems to prove, that in the progress of decomposition, the alkaline constituent has been partly carried off.

which are collected in reservoirs; then pumped into cisterns; and evaporated, either by natural or artificial heat. The beautiful white clay resulting from this process enters into the composition of the finest earthenware produced in this country. A good series of specimens, exhibiting this variety of moorstone in every stage of decomposition is now placed in our Geological Cabinets*.

The varied phenomena which I have attempted to describe, convey to the mind a most distinct idea of the constant degradation of some of the most solid parts on the surface of the globe. The milky appearance presented by many of the rivulets which trickle down from the moors, offer a striking illustration of this fact. We perceive these rivulets every instant carrying away the great constituent of the granite; distributing it in the lower regions in beds of plastic clay, or bearing it into the waters of the ocean.

We shall now proceed to describe some of the rocks which are usually associated with the granite. Among the most common varieties may be mentioned, 1" Those which arise from the addition of a fourth mineral, (as for example schorl or oxide of tin) so disseminated as to become a true constituent of the mass.

2. From the disappearance of one of the component parts. A beautiful example of this may be seen at the foot of Dartmoor between Ivy Bridge and Harford. The rock immediately in contact with the schist is composed of bright red felspar and quartz; presenting some traces of that aggregation which forms the variety called Graphic Granite.

3d. From one of the constituents being superseded by a new mineral. An aggregate of quartz, felspar, and earthy chlorite, is found in contact with the killas at the junction near Polmear Porth. A similar aggregate from the eastern side of Pra-sands has already been mentioned. A still more common rock is composed of quartz, felspar, and schorl.

See Granite, No. 60.—No. 65.

Schorl rock (that is, a granular compound of quartz and schorl) is by far the most striking of all the mineral masses associated with the granite. I shall here enumerate some of its most common varieties.

1. Granular quartz rock, with deeply striated prismatic crystals of schorl, of a coal black colour, and without regular terminations uniformly disseminated through the mass *. It is most commonly of finer texture than the granite; but varieties may be found, especially on the north side of Dartmoor, in which a very coarse granular base contains crystals of schorl several inches in length.

2. Where quartz prevails nearly to the exclusion of the schorl which only appears in small specks.

3. Where the schorl predominates; sometimes nearly massive; often in nodular corrections; which, when broken, exhibit a beautiful assemblage of divergent crystals. These concretions are sometimes more than a foot in diameter. This variety abounds on some of the moors to the south-west of Roach. Traces of schistose rocks are found immediately to the west of Roach; the schorl rock in question must therefore be placed near the junction of the granite and slate.

4. Where the schorl rock is porphyritic, containing fleshcoloured crystals of felspar. These crystals are often decomposing. In some instances they have disappeared altogether, and the base then resembles a scoria.

Specimens were obtained from the south side of Dartmoor, the moors near St. Stephen's and St. Austell, and the parish of St. Just.

Blocks presenting some of the above-mentioned varieties, lie

* In one instance I saw the striated crystals with regular terminations. They were obtained on the east side of Dartmoor, but I did not find them in situ.

scattered about the surface in many parts of Devonshire and Cornwall, especially near the junction of the killas and granite. They form the whole of that magnificent mass of rock near Roach ; and in that neighbourhood are so widely extended, as almost to assume the characters of a distinct formation. In general however we do not find them in situ; but they seem to be undecomposed fragments of veins, or irregular masses which were once imbedded in the granite. This opinion is confirmed by the appearance of the western cliffs, which are irregularly traversed by veins of schorl rock varying from the fraction of an inch to many feet in thickness.

Between Lemorna-Cove and Land's End; we not only meet with these schorl-rock veins, but with some other phenomena of which it is difficult to convey an adequate idea by mere description. Large irregular masses are observed to stand out from the cliffs, which at a distance have the appearance of dykes cutting the granite in a direction nearly perpendicular to the horizon. After a more minute examination, they seem to be the remains of irregular masses of contemporaneous rock, which project from the parts in which they were once imbedded, because their physical texture offers a longer continued resistance to the action of the elements. At their junction with the granite, the latter rock is always in an unusual state of decomposition, which has enabled the waters to cut out from the base of the cliff many caverns of strangely diversified forms. This anomalous rock is principally composed of fine granular felspar of grey, yellowish, or bright red colour, with a few scattered particles of quartz. Throughout its mass may be observed a number of dark spots, which are formed by stellated aggregations of schorl*.

The magnificent funnel-shaped cavern at Tol Pedn Penwith has been formed in the manner before described. The cliffs are there

* See Granite, No. 70, &c.

intersected by many vertical veins, some of which consist of a decomposing granite which readily yields to the action of the waters. One part of the cliffs has, in consequence, been undermined, and the cavern afterwards extended to the surface by the continued degradation of its crumbling roof. Among the veins of true granite are others of far different character, composed of large crystals of felspar of a brilliant red colour, occasionally mixed with divergent crystals of schorl. These veins are singularly tough and difficult of fracture, and obviously differ only in the colour and magnitude of their constituents from some varieties already described.

The enormous open work of Carglaze near St. Austell is an object of no ordinary interest. The traveller may there see the operation of the miner carried on in the light of day, without being compelled to descend a hundred fathoms below the surface of the earth and then to crawl into a dirty dripping cavern. The works are excavated in a variety of decomposing stanniferous granite or schorl rock. Its chief constituents are quartz, felspar, schorl, and oxide of tin, with occasional specks of mica. Throughout the whole extent of the excavation, we may trace a succession of parallel veins of schorl rock which do not in any degree partake of the decomposition of the metalliferous beds, and appear both in their range and dip to correspond exactly with the beds of killas which are seen in the immediate neighbourhood. This schorl rock at the immediate junction seemed to pass into the slate by almost insensible gradations. These phenomena induced me to believe that the whole mass was stratified; but subsequent observations have convinced me that the conclusion must have been

erroneous.

St. Michael's Mount is a fine instance of veined granite, and of the varieties of rock which arise from such a texture. It affords also a very striking illustration of the fact, that veins which have every appearance of being contemporaneous, often, for a consider