DOLERITES OF CORNWALL. 301 



Camborne a bluish-green dolerite, with a slaty structure, stands out 

 some 30 or 40 feet above the surface. It is hornblendic, and closely 

 resembles the hornblendic slates of Penzance ; and many other rocks 

 which have been classed as greenstones are also hornblendic, like the 

 beds at Newlyn East. At St. Stephens there is another blue elvan, 

 which has many of its felspar crj^stals replaced by schorl and cassi- 

 terite, but like all the other rocks has the chemical composition of 

 dolerite. I31ue elvans are only met with in the neighbourhood of 

 granite. 1 



A mile west of St. Austell there is a dolerite which extends in a 

 south-easterly direction to the sea near Duporth. It is about 90 feet 

 thick, and only preserves crystalline structure in the central part. 

 Occasionally orthoclase is present with the plagioclase ; the augite is 

 frequently replaced by hornblendic pseudomorphs. In the cliff 

 section at Duporth, the rock has the aspect of an aggregation of 

 boulders cemented by a mineral like asbestos. This is due to de- 

 composition along lines of fissure. Various other altered dolerites 

 occur at Tregorrick and Hallane. 



Another greenstone region stretches from Trevose Head on the 

 west to beyond Camelford on the east. Near the coast the rocks con- 

 sist of foliated ash beds, vesicular lavas, and augite lavas. When the 

 rock is vesicular it is often termed dunstone. At Pentire Point there 

 is a dark-green lava with abundant microliths of hornblende. The 

 dolerite at St. Tidy is composed of plagioclase, viridite, green horn- 

 blende, with minute garnets, a little apatite, and occasional grains of 

 quartz. 



The ancient lavas of Northern Cornwall have often a greenish- 

 grey colour and an amygdaloidal structure. At times amorphous, they 

 are frequently divided up into blocks by joints, and at other times 

 occur in foliated sheets, foliation evidently resulting from the move- 

 ment of the rock in a fluid state. The amygdaloidal lava of Pentire 

 Point contains 43 per cent, of silica. The cavities are generally filled 

 with crystalline calcite and viridite, or quartz and chlorite. Near 

 South Petherweir the dolerite is almost entirely unaltered. Near 

 Liskeard, dolerites are well developed ; and in South-East Cornwall 

 vesicular lavas again become plentiful. The transformation of the 

 augite into hornblende often begins with an external hornblendic 

 fringe, and at last the crystal is replaced by a mass of hornblende 

 microliths, but sometimes the augite becomes converted into uralite. 

 Mr. Phillips suggests that some of the slaty hornblendic rocks which 

 have the composition of dolerites may originally have been flows of 

 volcanic mud; such rocks are limited to "Western Cornwall. The 

 interstratified condition of the vesicular lavas admits of no question, 

 but the eruptive dolerites also are probably of the same age as the 

 strata in which they occur, because they do not traverse the granite, 

 but are disturbed by it. 2 



1 J. A. Phillips : Q. J. G. S., vol. xxxii. p. 155. 



2 J. A. Phillips : " On the so-called Greenstones of Central and Eastern 

 Cornwall," Q. J. G. S., vol. xxxiv. p. 471. 



