Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1027

Rh feet thick, forms a sort of terrace, some height above the shore, which can be reached with a little difficulty; the gabbro is extremely coarse, the diallage crystals being often two or three inches long; and some of the largest occur in a vein only a few inches thick. Just on the left there appears to be a small fault in the serpentine; on the right is a large included mass of schist forming a headland, down to which descends a dyke of dark compact trap about a foot wide. This can be seen to cut through two sheets of gabbro and the serpentine between them, and ends abruptly against the schist. The examination of this part of the coast leads to the following conclusions:—

1. That the serpentine is an intrusive rock.

2. That probably the hornblende schist was metamorphosed prior to its intrusion.

3. That the gabbro was probably intruded when the serpentine had arrived at its present condition.

4. That the black trap dyke was intruded last of all.

Under the microscope the trap is found to consist of a groundmass generally microcrystalline, consisting probably of some pseudomorphic product after plagioclase, with perhaps minute crystals of actinolite enlarged, and a large number of small hornblende crystals, merely rather platy in structure, but varying from the normal to the actinolitic form. There are, as usual, grains of magnetite, and a number of microliths, sometimes acicular, sometimes rather irregular in form, which are commonly included in the larger hornblende crystals, lying with their longer axes in the planes of principal cleavage. I have not been able to satisfy myself as to the nature of these. At present we must call the rock a diorite; but whether it has always been hornblendic is by no means certain.

From the above headland a walk of about a quarter of a mile along the edge of the cliffs leads to Polbarrow Cove. The cliffs,