482 J. J. Harris Teaii — The Lizard Gabbros. 



and Prof. Bonney. 1 Their mineralogical composition, geographical 

 distribution, and structural characters were described by the older 

 observers. Prof. Bonney added many facts on these points and 

 gave in addition a description of their microscopic structures. As 

 I wish on the present occasion to refer to them for the purpose 

 of establishing a definite proposition with regard to the origin of 

 certain structures which they possess, I must ask permission to 

 recapitulate the facts, and at the same time to add one or two from 

 my own observation. 



Gabbro occupies an area of six or seven square miles in the neigh- 

 bourhood of St. Keverne, and forms the elevated tract of land known 

 as Crousa Down. This great mass comes down to the sea and forms 

 the coast-line from Coverack to Manacle Point. It is traversed by 

 veins and dykes of a fine-grained rock generally known as " green- 

 stone." This greenstone is essentially composed of plagioclase and 

 hornblende, the latter mineral often belonging to the uralitic and 

 actinolitic varieties. It is, as Prof. Bonney has pointed out, an 

 altered plagioclase-augite rock, and must be classed with the 

 epidiorites (Gumbel) of the Fichtelgebirge and the Hartz. In some 

 instances the felspars give lath-shaped sections which penetrate the 

 fibrous aggregates of secondary hornblende in such a way as to 

 prove conclusively that the parent rock must have been an ophitic 

 dolerite. 



The amount of this " greenstone " or " epidiorite " associated with 

 the gabbro increases towards the north until it exceeds that of the 

 latter rock. Although the junctions of the gabbro and the green- 

 stone are perfectly sharp, it is impossible to represent the distribution 

 of the two varieties on a small scale map in consequence of the way 

 in which they alternate with each other. Manacle Point, which is 

 represented as greenstone on the Survey Map, consists for the most 

 part of gabbro in which veins and dykes of greenstone are very 

 common. 



The northern limit of the gabbro-greenstone area above referred 

 to occurs in the neighbourhood of Porthoustock. The adjacent rock 

 is hornblende-schist, but the actual junction, which is probably a 

 fault, is concealed by a small shingle beach. The southern limit 

 of this mass comes out on the sea-shore by the small village of 

 Coverack. The geology of this locality has been fully described by 

 Prof. Bonney, to whose paper reference must be made for details. 

 The only point that need be mentioned here is that the normal gabbro 

 is seen to be intrusive in serpentine and also in a rock, intimately 

 associated with the serpentine, which consisted originally of a basic 

 felspar and olivine and which appears to be identical with the trocto- 

 lite or forellenstein of Neurode in Silesia. 



South of Coverack the greater part of the district is occupied by 

 serpentine in which veins and dykes of gabbro frequently occur. 

 The mass of gabbro which ranks next in importance to that of Crousa 

 Down is exposed on the sea-coast west of Lankidden Cove. It forms 

 the headland of Karakclews, and may be followed along the coast 

 1 Quart. Journ. Geol. Soc, vol. xxxiii. 1877, p. 884. 



