486 



PRO J?'. T. G. BONNE Y AND MAJOR-GEN. C. A. M*=MAHON 



or banded structure, one with another and with the exterior of the 

 mass, will best appear from a series of examples. 



(a) In the Carrick-Luz dyke the strike of the structure appears 

 to ibe fairly steady *, running rather W. of N.W., which is also the 

 average direction of the dyke. It appears to dip at a rather high 

 angle, 60° or more, on the northern side — that is, in the probable 

 direction of the fissure. The mass varies somewhat in coarseness, 

 and in the amount of foliation and of banding, the latter being more 

 distinct on the western side, but here and there it seems in- 

 conspicuous. The felspar occasionally, the diallage frequently, are 

 unaltered. Now serpentine yields readily to pressure. When this 

 is moderate in amount the rock brecciates ; when the pressure is 

 more severe the fragments take a lenticular shape and become 

 slickensided ; when it is extreme, as may often be seen in the Alps, 

 the rock assumes the appearance of a slaty schist, with correspond- 

 ing changes in its microscopic structure f. Any conspicuous grains 

 of bastite, augite, chromite, &c. are more or less crushed out. But 

 in the Lizard serpentine these minerals, like the matrix, are in a 

 normal condition, so that we are forced to conclude, if we adopt the 

 pressure hjj-pothesis, that the gabbro, one of the toughest of rocks, 

 has been crushed into a kind of schist, while its comparatively 

 brittle associate has undergone no structural change. 



(b) Sometimes the foliated structure in the gabbro occurs near to 

 and parallel with the edge of a dyke, but at others, though rarely, 

 it is inclined at a high angle to it; it is also found in wisps or 

 streaks in a non-foliated mass, as may be seen, for instance, at Crousa 

 Down. The annexed diagram (fig. 5), representing part of a vein 

 on the eastern side of Compass Cove, shows a wedge of serpentine 



Pig. 5. — Foliation of gabbro dylce east of Compass Cove. 



A B 





D. Serpentine. 

 A, B. Moderately fine gabbro, with foliation; from A to B about 

 4^ inches. 

 C. Coarse Gabbro. 



* We can answer only for the shore-section. 

 t Bonney, Geol. Mag! (1890) p. 533. 



