Trewavas Head, in the parish of Breage, in Cornwall, few of which, in my opinion, can be clearly understood by those who have never visited the spot, consequently less likely are they to be able to decide on their disputed nature, whether the granite composing the veins is of the primitive or secondary formation. Having very recently visited the spot, and taken Mr. Sedgwick's description of these veins with me, I find the account given by him nearly correct; at the same time I discover that he has omitted to notice some circumstances which might tend to elucidate, in a more correct manner, the nature of their formation. In endeavouring to supply this deficiency, I have thought it advisable to give a section of the cliff, or an outline of its appearance from the beach at low water (Pl. XXI), fig. 1, and add a few observations which I conceive necessary as we proceed in his description.

"About a quarter of a mile east of Trewavas Point (and about 100 yards east of this sketch), where the cliffs are in an unusually ruinous state, a small brook has excavated a passage to the water's edge. The killas rocks on the beach appear to be intersected by numerous contemporaneous veins of quartz. Near this spot several thin beds of granite seem to alternate with the slate; one in particular, which preserves its thickness and conformity to the lamina of the schist for upwards of 100 feet, when it is lost in the waters." The slate lying both above and below this granitic vein as it traverses the beach, is washed from its surfaces, so as to leave it projecting in many places several feet, so that its dip is very visible, and is found to be as in the cliff at about an angle of 28°. "However, a further examination," says Mr. Sedgwick, "discovered its real nature; for upon observing it in an opposite direction, a number of smaller veins were seen emanating from it. It then cut obliquely through the lamina of slate, starting off from its first direction, and became finally lost in a waving line among the cliffs. The greatest width of this vein is about two feet, and its extent from the edge of the water to its termination in the cliff is about 400 feet.

"Further west, the granite veins are crossed by two others of a different character; one of them ranges nearly in the magnetic meridian, and underlies east two feet in a fathom; the other underlies in an opposite direction. They are about a foot and half in width, and contain quartz, oxide of iron, and a little clay slate." This quartz vein ranging nearly in the magnetic meridian, produces upon the granite vein, the same effect that cross courses often have upon metalliferous veins in most of our mines, that of heaving it out of its direct course. Here, fig. 2, the granite vein is heaved up about three feet by being intersected by the quartz vein; while another quartz vein, a few feet distant, is seen pursuing its regular course, being interrupted only by the granite vein. This circumstance I shall have occasion to remark more fully hereafter.

"For a considerable extent beyond this point, the whole base of the cliffs is covered with vast fragments of the veins which have been denuded by the surrounded killas becoming decomposed; one of these is 10 feet thick. In general they are of a brilliant white colour, and of a fine granular texture, sometimes containing within themselves parallel veins composed of large crystals of quartz and felspar, and proved to be of contemporaneous origin by the long spiculæ of schorl which pass without interruption, through both the quartz and felspar."

These coarse granitic veins within the granite are best seen in many of the huge blocks on the beach; one block in particular, I observed, that has one of its sides nine feet long, and seven broad, covered with these immense crystals of quartz and felspar, and which most probably had separated from its fellow, by the fall from the cliff. One crystal of felspar I separated which measured 44 inches in diameter These coarse veins, generally speaking, are not more than from four to eight inches thick; but much of the granite apparently forming the matrix of the beach, in this place, seem to be wholly composed of these large crystals, in which is found some schorl, and scarcely any mica; while other parts of the granite have merely the large felspar crystals imbedded in it, as to render it completely porphyritic. One block of considerable magnitude has a vein of deep coloured amethyst passing through it, several small crystals of which I collected.

"Beyond the ruin of these veins, there is a bed of granite one foot thick, and about 40 feet in length and breadth." This is the coarse-grained granite just alluded to, but it varies in thickness from one to five feet. This " passes under the cliff, and to all appearance alternating with the slate, but which, as in the former instance, turns out to be a granitic vein. Advancing further to the west, the rocks are beautifully intersected with veins of the like nature, the lower part being cut through by a well defined vein of about a foot thick, while the higher parts are traversed by innumerable ramifications; the lower branch after keeping the direction of the slate beds, for a distance of 60 feet, suddenly rises in a perpendicular direction to the top of the cliff. The whole of this system of veins afterwards unite in one trunk, which after traversing a projecting ledge of rocks, descends in an oblique direction into a great mass of granite, which form a part of a natural cavern. this spot appears a very large mass of granite, which seem to be the root of the gigantic veins, which proceed from this point, and rise in broad white lines towards that part of the cliff which reposes immediately on the central granite. Splinters of clay slate are here seen imbedded in the middle of the granitic veins.

Near

"From this point two large veins separated by a lancetshaped mass of slate, rise towards the west at an angle of about

15°. Within a few feet of these two, a third vein starts out at nearly the same angle, and proceeds in the same direction. These three veins are throughout nearly of the same thickness, viz. each about five feet.'

Whether the recent fall from the cliff during the last winter has altered the features of the veins, or exposed a new one, I cannot state with certainty; but there is distinctly to be seen at present four separate veins as represented in the section; the lowest is not more than three feet thick, until it arrives at the point (a), when it suddenly widens to more than six feet, at which thickness it continues on to the west. The vein immediately above this (b) commences about five feet in thickness, and continues on at the same width as far as immediately over the widened part of the vein below, where it decreases to about 24 feet, and so continues on to the recess. These two are at about an angle of 15°. The next vein (c) is about seven feet thick throughout, and rises at about an angle of 30°, and it is in this vein principally where the fragments of slate are so very conspicuous: some of these fragments measured from three to four feet in length, and from four to six inches thick: they show themselves in the veins in the manner represented in fig. 3. At other places the slate may be seen apparently shooting into the veins of granite in a tortuous manner, fig. 4: a fourth vein (d), about eight feet thick, is found rising at an angle of about 45° till it is lost in the alluvial soil above. This vein does not appear to be noticed by Mr. Sedgwick.

On examining some of the rocks lately fallen, many fragments of slate are to be found imbedded in the granite, and several masses of slate may be seen with granite adhering to one or more of its sides, and so firmly attached to it, that the granitic vein itself has split in preference to separating from the slate.

"The two lowest veins preserve their course without being much deflected for some hundred feet, and from the place we first remarked them, disappear behind a projecting part of the cliff. On turning this projecting ledge, we suddenly reached a recess, the lower part of which was filled with the ruins from the higher of the overhanging rocks. The western side of this recess is composed of killas, intersected by some small granitic veins." About half of the western side only is composed of killas; close to the alluvial soil is granite 15 feet thick; then comes a thin layer of slate about three feet thick, which is again followed by a granitic vein (g), about 15 feet in thickness. The remaining part of the cliff below is all slate, which entirely disappears about 200 feet further west than this recess. In this last described slate are to be seen blocks of rounded granite, or what in other situations would be called bowlders of small dimensions imbedded in the centre of the slate as seen at (e). This granite has a different aspect from any other in the immediate neighbourhood, being of a darker and firmer texture, and