400 ME. r. EUTLET Olf EETIPTIVE EOCKS EEOM THE 



stitution is concerned), or what, a few years ago, we should have 

 called a basalt or dolerite; and if a name based upon colour be 

 worth anything, it is also a very good greenstone, and as such it was 

 formerly mapped by the Geological Survey. The other exposures of 

 eruptive rock in its immediate neighbourhood have, however, been 

 coloured alike ; but, although we can see at a glance that the 

 rocks of Cant Hill and Carlion are not of precisely the same character, 

 there is a certain rough justice in this generalization, which was 

 absolutely necessary in the early days of the Geological Survey, 

 since both are rocks which apparently possessed a somewhat similar 

 mineral constitution in the first instance. In the case of rocks which 

 present such frequently ambiguous characters as those of Cant Hill, 

 the work of the microscopist would be more sure if controlled by 

 observations made in the field; but Cant Hill to-day is probably 

 much the same as, it was when the Survey mapped it, and there 

 seems little more to be learnt until fresh quarries are opened out. 



There are, however, probably points concerning the relation of the 

 eruptive rocks to the slates which would well repay further investi- 

 gation. If the foregoing remarks should throw any faint glimmer 

 of light on the nature of the rocks at Cant Hill and Carlion, I trust 

 that they may be accepted rather as a slight addition which Sir Henry 

 He la Beche would have made to his own " Geological E-eport on 

 Cornwall," than as an attempt to correct the work of one of our old 

 masters. 



EXPLANATION OF PLATE XII. 



Fig. 1. Devitrifiecl basic lava, showing fluxion- structure and vesicles, A large- 

 proportion of this section consists of a clear, greenish-yellow to 

 brownish-yellow serpentine. The. opaque central band is probably 

 an altered spheralitie structure. 

 Cant Hill, near St. Minver, Cornwall ; east end, at top of hill. 

 Specimen no. 1, X50. Ordinary transmitted light. 



2. Devitrified basic lava, showing portion of the original glass, slightly 



altered, a fibrous structure being visible in it when seen between 

 crossed nicols. The extremely vesicular character of the glass is 

 shown in this drawing. The greenish granular matter in the upper 

 right-hand corner is probably epidote. 



Cant Hill ; west end, at top of hill. 



Specimen no. 3, X 50. Ordinary transmitted light. 



3. Devitrified basic lava, showing porphyritic crystals of felspar, some of 



which appear corroded, and numerous small vesicles filled with quarts 



and serpentine. 

 Cant Hill ; east end, at top of hill. 

 Specimen no. 2, X 25. Ordinary transmitted light. 



Discussion". 



The President remarked on the characteristic treatment by the- 

 Author of a difficult subject. He had laid many sides of the question 

 before the Society with the greatest candour. 



Mr. Teall remarked that the specimens from Cant Hill were 



viz. andesite and porphyrite. In its i-elation to structure the latter term is> 

 preferable when a ground-mass exists. — F. E., June 28, 18(56. 



