Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1036

906 schistose structure. The top and bottom of the rock shows the usual mineral changes. The serpentine in contact is much cracked and burnt. The projection of rock, which forms the western limit of the port is cut by a dyke of newer gabbro; and the same rock, a few yards further on, appears at the base of the cliff on which the village stands; an isolated patch also breaks through the serpentine on the shore. A few yards from this is the first patch of the older gabbro. This older gabbro forms the base of the cliff for a short distance, while the shore consists mainly of newer gabbro, veins of which cut the older. Just where a projecting angle of the cliff begins is an interesting junction. The older gabbro is cut by a dyke of newer gabbro about 18 inches wide, and both again cut by a dark trap dyke about 12 inches wide (fig. 6). The two intrusive dykes are displaced by a fault of a few inches. This last dyke (1) may be traced for some distance towards the sea; its general direction is N.N.E.–S.S.W. At the angle named above, serpentine replaces the newer gabbro, both on the shore and in the cliff, and on the whole is continued to beyond another small projection.

Here another trap dyke (2), generally about 5 inches wide, may be traced about 20 yards along the shore in a northerly direction. Beyond this veins of the newer gabbro break repeatedly through the serpentine. Then comes a dyke of compact trap (3), about 4 inches wide, cutting into the cliff and running N.N.W.–S.S.E. Beyond we find serpentine with intrusions of newer gabbro, till we again find the older gabbro beneath the cliff, broken into by the newer, and both cut by another compact trap dyke about 1 foot wide. A few yards further, over serpentine and newer gabbro, is another trap dyke cutting both, exposed up the face of the cliff for 7 or 8 feet. It is about 3 feet wide, and rather coarser in texture than the others. Well-marked horizontal joints give it a slightly columnar aspect. The shore for some little distance further consists of serpentine with some intrusive newer gabbro, and at least one more dyke of compact trap.

It will now be convenient to describe more precisely the lithological characteristics of these rocks, omitting at present the serpentine.

The Older Gabbro.—This might easily be mistaken for a mere variety of serpentine. It has a compact, dark, dull red or purple groundmass, often mottled with a brighter red, in which are imbedded crystalline grains of a greyish white felspar, perhaps about 0⋅2 inch in diameter, and some rather smaller crystals of diallage, generally about $1⁄4$ to $1⁄3$ inch apart. The matrix for a quarter of an inch square or more is often unbroken by any crystals visible to the eye; occasionally, however, the felspar predominates. The rock does not vary much in texture, the smaller veins being about as coarse as the larger masses. One small boss has the groundmass a dark green instead of red. It is, however, the same rock, though as a rule it is rather more finely crystallized than the average of the