GEOLOGY 



which occupy the rest of Cornwall. The transition between the respec- 

 tive rock types is sudden, and the line of demarcation appears also to 

 be a line of dislocation. The precise relations therefore of the killas 

 and the Lizard rocks have not been determined. 



The dominant rock in the Lizard peninsula is undoubtedly ser- 

 pentine, which covers an area of about 20 square miles, while asso- 

 ciated with it are considerable masses of gabbro and hornblende schist. 

 Granite and greenstone (epidiorite) are of subordinate occurrence ; while 

 the extreme south of the peninsula is occupied by schists and gneisses. 



The serpentine extends from Lizard Town on the south to Polwin 

 in the north. It is seen along the east coast from Coverack Cove to the 

 bay below the village of Landewednack ; and on the west coast between 

 Pentreath Beach and George's Cove, and further north at Mullion Cove. 

 Smaller masses occur in the vicinity of Porthalla. 



The variety of the Lizard serpentine constitutes one of its most 

 striking characteristics. This is largely due to the fact that the rock is a 

 product of alteration of pre-existing rock masses ; not only did the latter 

 differ in mineral constitution, but the extent of their decomposition, which 

 has given rise to the serpentine, has not been uniform. While in some 

 instances the metamorphism has been complete, in others the original 

 rock is still represented by portions that have been undestroyed. 



The serpentine has been mainly derived, as shown by Professor 

 Bonney, from olivine bearing rocks, the decomposition of that mineral 

 especially favouring its production. Moreover, other magnesian minerals 

 pass into serpentine in a similar manner. According to Professor 

 Bonney, the handsome mottled varieties, with conspicuous crystals of 

 bronzite occur chiefly in the southern and south-eastern regions, along 

 the coast from the Balk to Coverack Cove, and as far inland as the 

 neighbourhood of Ruan Major : the fine black variety with glittering 

 bronzite crystals being found on the coast south of Caerleon Cove ; as 

 also on the west coast to the north of Kynance Cove (near the Horse). 

 South of that and at the Rill, as well as to the north at Gue Graze, George 

 Cove and near Mullion Cove, compact dull-coloured varieties are com- 

 moner, and these are seen by the Helston Road on the northern part of 

 the mass. In close association with these is the dull-coloured variety, 

 containing small glittering crystals, which appear to be mainly a pyrox- 

 enic or hornblendic mineral (grammatite), found especially about Lower 

 Pradanack and in Mullion Cove. 1 



A small patch of serpentine also occurs (beyond the Lizard region) 

 in Gerrans Bay near the Nare Head. This rock varies from a massive 

 to a sheared serpentine, and like the serpentine of the Lizard occurs in 

 association with gabbro. The relations however of the serpentine and 

 gabbro to the ' killas ' are obscure, as neither of the former reaches the 

 coast. 



Next in importance to the serpentine are the large intrusive 

 masses of gabbro; moreover smaller veins of gabbro pierce the ser- 



1 >uart. Journ. Geol. Sac. xxxix. 23. 

 43 



