1877.] Mr Bonney, On the rochs of the Lizard District. 85 



consolidated. That the pebbles may have crumbled down in 

 this way is proved by the fact that the ordinary syenite whenever 

 it comes to the surface is covered with a considerable depth of 

 a rotten material of a similar character, and exposed lumps of 

 syenite weather down to a rounded form resembling rolled 

 pebbles. 



The general conclusions to which my observations of these 

 rocks point are, that the granitic structure is a metamorphic 

 character which may be imparted either to stratified or to 

 igneous rocks ; but is not due to igneous fusion, or to any 

 action which can occur only at a great depth in the earth. The 

 names "plutonic" and "hypogene" applied to granitic rocks 

 may be excellent on the lucus a non lucendo principle, biit such 

 rocks ought to be classed as metamorphic, and their igneous or 

 aqueous origin determined by consid'Crations entirely different from 

 their crystalline characters. 



A communication was made to the Society by 



(2) Mr BoNNEY, On the RocTcs of the Lizard District ( Cornwall). 



The author brought forward evidence to prove that the serpen- 

 tine of this district was clearly intrusive among the hornblende 

 schists. He then described the microscopic structure of the ser- 

 pentine, and showed by comparison with the olivine of Iherzolite, 

 and of certain gabbros, that it was an altered olivine rock. The 

 gradual conversion of olivine to serpentine, as exhibited in these 

 instances and in slides selected from the Lizard rocks, was 

 described. Enstatite and augite were frequent in the latter, with 

 iron peroxides, as well as (occasionally) picotite. Hence the Lizard 

 serpentine may be regarded as an altered Iherzolite. The gabbros, 

 which are intrusive in the serpentine, were then described. These 

 are of two ages ; the older, probably limited to Coverack Cove, has 

 olivine partially converted into serpentine, but is otherwise not 

 greatly changed. The newer (for that of Crousa Down is probably 

 of the same age as the veins on the east coast) often has its 

 pyroxenic constituent, and apparently the olivine also, to a large 

 extent converted into minute hornblende, and its felspar into a 

 hydrous mineral, which may be called saussurite. A brief descrip- 

 tion was given of the granite veins of the west coast, and of some 

 metamorphic rocks on the east coast resembling granite. On the 

 latter coast are found dykes of dark trap, the iiewest rocks of all ; 

 some are augitic, some hornblendic ; but the writer thought that the 

 latter mineral was probably an alteration product. 



