484 PROF. T. G. BONNE Y AND MAJOR-GEN. C. A. M^MAHON 



to be connected with any mode of dynamo-metamorphism. It is 

 quite true that it is very characteristic of the foliated masses, but it 

 may also be observed, as, for instance, at Coverack, in rock of the 

 most normal character. It is evidently due to the action of water, 

 and might more correctly be designated meteoric metamorphism, for 

 it evidently proceeds inwards from the exterior of the mass ; pro- 

 bably being produced when this is at no great distance from the 

 surface. 



This gabbro occasionally is distinctly foliated or even banded, a 

 structure which during the last few years has given rise to much 

 discussion. Prof. Bonney, in describing it, regarded the structure as 

 the result of crystallization under a pressure (or resistance) definite in 

 direction. Mr. Teall ascribed it to pressure subsequent to solidifi- 

 cation *, and compared it with the ^as^r-gabbro of the Germans, 

 which has been similarly explained. But to this view, as pointed out 

 by the former t, and subsequently confirmed by Gen. M^MahonJ, 

 the absence of all signs of the effects of pressure in the associated 

 serpentine seems a fatal objection. 



One of the chief objects of our visit in 1890 was to study afresh 

 this very remarkable structure, and the conclusion at which we 

 arrived will be most readily indicated by giving a brief summary of 

 our observations, though this may involve some slight repetition of 

 statements already published. 



The foliated and banded structure in the gabbro is most con- 

 spicuous in the Carrick-Luz dyke ; it is also locally very weU 

 developed in the neighbouring dykes, especially on the western side, 

 in the neighbourhood of Pen Voose and at Polbarrow. It may be 

 observed, though it is not common, in the Crousa-Dovvn mass, and 

 elsewhere. Every variety may be found, from a slightly streaky or 

 wavy foliation § to a distinct mineral banding, and not seldom the 

 pyroxenic crystals appear as " eyes." Each of these two types is 

 excellently figured by Mr. Teall [j, so that on the present occasion it 

 is needless to do more than refer to his plates and to the descrip- 

 tions already published. The results of our investigations may be 

 thus summarized : — 



(1) As will afterwards be more fully explained, the gabbro had 

 assumed its foliated structure before it was cut by the later intrusives 

 (dykes of diabase, &c.), which probably are of more than one age. 



(2) Whatever be the origin of the quasi-foliated structure in the 

 serpentine, this cannot be connected with the foliation of the gabbro. 

 The former is most marked on the western coast and at Porthalla ; 

 but these cases cannot be cited in support of the pressure-hypo- 

 thesis, because gabbro does not, so far as is known, occur in either 



* Geol. Mag. (1886) p. 481. 



t Ibid. (1886) p. 575. \ Ibid. (1887) p. 74. 



§ We use this terai as indicative of orientation rather than of linear aggre- 

 gation of constituents, prodacing at most a slight and interrupted * streakiness,' 

 from which every stage exists to distinct bands mainly of different minerals, 

 sometimes over a quarter of an inch thick. 



|[ ' Brit. Petrogr.* pis. xxvi. and xliii. 



