240 



morphosed condition in East Cornwall and Devon. The most 

 common rock is a normal ophitic diabase. 



(3) Near the granite masses the diabases are represented by proterobases 



and epidiorites. Traces of an original ophitic structure are often 

 preserved in the metamorphosed rocks. 



(4) Tourmaline, pale brown mica and axinite are contact minerals. 



(5) Diabases, proterobases and epidiorites often show traces of 



schistosity. 



(6) This schistosity is mainly if not entirely the result of the post- 



Carboniferous earth movements. 



(7) The proterobases and epidiorites do not vary very much in chemical 



composition or specific gravity. 



(8) The foliated hornblendic rocks belong to two groups. The rocks of 



one group resemble the proterobases and epidiorites in composition 

 and specific gravity, the rocks of the other group are more basic 

 in composition, and possess a higher specific gravity. 

 The Lizard district is separated geologically from the rest of Cornwall 

 by a line of disturbance running from Porthalla on the east to Polurrian 

 on the west. We are not able at present to speak positively as to the age 

 of the rocks occurring on the south side of this line, or as to the date of their 

 metamorphism. In the eastern portion of the district between Porthou- 

 stock and Landewednack innumerable dykes of "greenstone" occur in the 

 gabbro and serpentine. They appear, however, to be entirely absent from 

 the hornblende-schist. In the south-western portion of the district there is 

 a zone of intense mechanical metamorphism and in this zone we find 

 lenticles and bosses of greenstone associated with actinolitic and horn- 

 blendic schists which owe their origin, in part at least, to the metamorphosis 

 of the "greenstone." The dykes on the eastern coast have been described by 

 Prof. BONNEY who arrived at the conclusion that " they were once all 

 dolerites or basalts, and that the hornblende which undoubtedly character- 

 izes many of them is a secondary product due to metamorphism of the 

 pyroxenic original constituent.''^ 



Subsequent research has merely tended to confirm this generalization, 

 at any rate so far as the majority of the dykes are concerned. 

 Macroscopically the rocks of these dykes are of a dark purplish or 

 greenish colour and finely crystalline. They frequently contain porphyritic 

 crystals of plagioclase. Granular and porphyritic specimens may, however, 

 be obtained from one and the same dyke. Porphyritic augites have not 

 been observed. When examined with the microscope the rocks may be 

 classed as dolerites, proterobases and epidiorites according to the more 

 or less complete replacement of the original pyroxene and olivine by 

 secondary hornblende. In some of the dykes foliation has been super- 

 induced by dynamic metamorphism, and in these cases the rocks may be 

 described as foliated proterobases, epidiorites, or even hornblende-schists. 

 Prof. BONNET calls attention to a remarkable dyke occurring at the 



(1) Q. J. G. S. Vol. XXXIII. 1877, p. 915 



