Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1015

Rh in mineral composition. As neither of these works is now very readily accessible, and I wish to make my paper as complete as possible, I have occasionally described phenomena noticed by them, although it has added slightly to the length of the communication. The microscope has, I hope, enabled me to explain several of those phenomena with which the appliances at their command could not deal. Where I differ from them, as I do occasionally, it is with diffidence, and only because, after careful consideration, I am unable to adopt any other conclusion. The remaining works are generally too vague in their terminology to be of much use at the present day.

For the general features of the Lizard peninsula I must refer, for the sake of brevity, to Sir H. De la Beche's admirable memoir; suffice it to say here that the district of which I more particularly treat may be described as a plateau, partly cultivated, partly wild moorland, which, as a rule, descends precipitously to the sea, except where it is furrowed by small coves and gullies. The cliffs often rise vertically from 100 to 200 feet above the sea; of beach beyond the high-tide mark there is but little, the base of the cliffs being then often washed by the waves for hundreds of yards together. In not a few places the cliffs are totally impracticable from above, and could only be examined (without a hope of landing) from a boat below in the calmest weather. This, of course, adds greatly to the difficulty of investigating the district; for, even at low water, progress at the base of the cliffs, where possible, is often laborious, and the state of the tide has to be carefully watched.

The district described in this paper forms the southern part of the Lizard peninsula. I have examined the coast of this from Lizard Head to Mullion Cove on the west side, and from the same to Manacle Point on the east, as carefully as circumstances admitted, and have also traversed the interior in two or three directions.

The following rocks, the positions of which may be seen on the geological map, occur in the above district:—(1) hornblende schist; (2) serpentine; (3) gabbro; (4) granite (restricted, as will be shown, to the west coast); (5) greenstone, i. e. dark augitic or hornblendic traps, described hereafter in detail, restricted, so far as I know, to the east coast.

The most convenient arrangement will be to commence with a few remarks on the hornblende schist, next to describe the petrology and stratigraphy of the two coast-sections in detail, and then the inland sections, reserving to the end all details of the microscopic structure of the serpentine.

The Hornblende Schist.—Under this title are included an extensive group of rocks which I do not profess to have minutely investigated, as I was chiefly occupied with the rocks of presumed igneous origin. The following description, however, will be fairly accurate:—This