A HISTORY OF CORNWALL
pentine. A large mass of gabbro forms the cliffs between Coverack and Manacle Point, and extends inland for about 4 miles as an ovate mass occupying an area of 6 or 7 square miles, including the districts of St. Keverne and Crousa Downs. A smaller mass ¼ mile in width extends inland from the headland of Karakclews to Gwinter. Another considerable exposure occurs on the shore near Landewednack.
According to Mr. Teall the principal constituents of the Lizard gabbro are plagioclase, augite or diallage, hornblende, olivine and saussurite, while original iron ores are rare or altogether absent.
The dioritic rocks that occur in this region appear as veins in the gabbro, and as part of a banded series consisting mainly of diorite and a fine grained granite. In places the two latter types vein each other, while they sometimes occur together as parallel bands. Mr. Teall describes the constituents of the dioritic rocks as felspar, biotite, green hornblende, sphene, iron ores and apatite.
The granite of the Lizard peninsula occurs as veins in the serpentine, gabbro and dioritic rocks, and is of different character from the Cornish granite already described from beyond that region. That of the Lizard has been deformed into schist. It veins the serpentine at Kynance, a dioritic rock at Pen Voose, and occurs in a gabbro north of the last-named locality, and also veins a diorite at Kennack Cove.
Hornblende schist forms a wide band at the northern edge of the Lizard complex and appears to extend practically right across the peninsula from sea to sea. This rock differs from the hornblendic rocks described from beyond the Lizard area in the frequent development of a highly banded character, which is so perfect that viewed from a short distance the white and green striping, representing the differentiation of the felspathic and hornblendic minerals, strikes the eye as a parallel arrangement of almost mathematical precision.
Between Porthoustock and Landewednack greenstone dykes occur in the gabbro and serpentine, ranging from epidiorite to hornblende schist, while in some a certain amount of pyroxene still remains. But in the neighbourhood of Coverack there are found dykes of olivine dolerite which have retained their original character.
In the southern portion of the peninsula there is a zone of fine micaceous and actinolitic schists which in the outlying islands have reached a still further stage of metamorphism, and as pointed out by Mr. Howard Fox consist of typical gneiss. As remarked by Mr. Teall this zone of intense mechanical metamorphism includes lenticles and bosses of greenstone associated with actinolitic and hornblende schists, the latter having originated in part at least from the metamorphism of the former.
In recent years keen controversies have arisen among geologists regarding the origin of the foliated igneous rocks and the relative ages of the serpentine and hornblende schists. On one hand it is contended that the foliation of the gabbro and the passage of dykes of porphyritic epidiorite into hornblende schist have been produced by dynamic metamor-
44
