Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1022

892 schist by the track which ascends it; the adjoining islands appear to be one serpentine, the other schist.

The hornblende schist on each side of the cove has a fairly regular strike, of about S. 23° W., running straight at the main mass of serpentine, and dips at a high angle (about 86°) on the western side; north of the above spot I could not find any more serpentine.

(a) The Coast to Coverack.—The sections on this coast are, on the whole, more complicated than those on the western. Commencing at the narrow cove of Perranvose near Landewednack, we find this cut down to the shore through hornblende schist; but by turning aside along a track before the steepest part of the descent, we are in a few minutes brought to a serpentine-quarry at the top of the cliffs above the sea.

By the side of the road leading into this, the junction of the serpentine with the hornblende schist is well seen. The former overlies the latter, following very nearly the plane of its bedding, which here dips at about 40°. The last foot or so of the serpentine is extremely rotten, crumbling into dust under the fingers; the hornblende slate is also rotten, and stained red; hence, as is often the case, the junction may be easily overlooked. Its nature, however, is clear; for careful search will detect two small tongues of serpentine a few yards distant in the hornblende slate—the larger about 2 feet in diameter. They are extremely rotten, but undoubtedly are serpentine.

The quarry furnishes some handsome varieties of this rock, generally a compact purplish or reddish brown groundmass, mottled and veined with pistachio green, in which are small groups of bronzite crystals of greenish hue and submetallic lustre, and sometimes veins of brightish red hæmatite (no. 9). Some masses are almost wholly of a grey-green colour; but this is only the result of decomposition; in some, minute crystals of magnetite are common. The lower part often exhibits a curious fissile structure, the cracks being filled by calcareous films. Sometimes the latter are about $1⁄4$ inch thick and the mineral is aragonite.

By descending the "tip" of the quarry to the beach and walking to the end of a little headland, we have further proof of the intrusive character of the serpentine. The headland consists of hornblende schist, resting on serpentine which forms a little isthmus; the slope on the south of the "tip" shews hornblende schist with intrusive tongues of serpentine; then two or three more masses of hornblende schist crop out in the slope; and finally there is a small headland of that rock. On the other side, beyond the debris, is serpentine curving round to another little headland of the same rock.

Rounding this, we enter a second little recess, and passing two fragments of schist included in the serpentine, come, on the opposite side, to a third of the most singular shape. Any one looking at the outline only, would take it for a dyke (see fig. 1). The bedding however,