VOLCANIC ROCKS OF CORNWALL. 



By THOS. CLARK. 



Olivine appears to be generally considered a truly volcanic 

 production. It is found in ancient and modern basalts, lavas, 

 and kindred erupted rocks of Egypt, Natoba, Brazils, Styria, 

 Vesuvius, Mexico, Sweden, and Baden, it is also common in 

 the basaltic green-stones of Scotland, and is now found in 

 Cornwall. It consists of about 40 per cent. Silica, 48 Magnesia, 

 1 1 Iron Protoxide with traces of Manganese and Alumina. 



Its crystals are rhombic prisms, it colour varies from bottle 

 or olive green to brownish or yellowish. Its hardness varies 

 from 7 downwards, according to its state of decomposition ; it is 

 soluble in hydrochloric or sulphuric acid, and its decomposed 

 matter sometimes goes to form other crystals. Serpentine is 

 generally accepted as a rock highly impregnated or cemented 

 with a solution of olivine. Prof. Bonney first recognized it in 

 some serpentine of the Lizard about 6 years ago, but instead of 

 what he found being an accidental crystal, it is the reverse, for it 

 is so abundant in the serpentine of the Lizard that I may safely 

 say over 90 per cent, of it is unworkable for ornamental or 

 decorative purpose, on account of its presence, and the small 

 portion of it that is workable contains mild pseudomorphs after 

 Olivine. Within the past few years I have been working on 

 these rocks, and made some discoveries of this mineral in the 

 Lizard, Duporth, and Menheniot serpentines. I have also 

 discovered other volcanic evidences in a trap rock in the parish 

 of Kea, by the Truro river, between Tregothnan boat-house and 

 Cowlings creek. Hand and microscopic specimens of the various 

 rocks I have placed in the Museum, for comparison with 

 those from Vesuvius. The specimens from Kea are from a 

 micacious trap, whose out-crop is visible in many places 

 between two volcanic regions, viz : — the Lizard and near 

 Padstow. This trap has burst through the lower and upper 

 Silurian between the Lizard and Newquay Head ; at Padstow 



