ON TOE CKTSTALLINE ROCKS OF THE LIZARD DISTRICT. 489 



igneous rocks. The diorito is certainly not derived, in this case, 

 from the gabbro by sliearing, and its intrusion was subsequent to 

 the epoch wlien tlie gabbro had attained a maximum foliation. 



(«) One more typical case may be given. On the top of the cliff's 

 above Tolbarrow there is a boss of gabbro perfectly unfoliated. A 

 few feet below this a vein of gabbro appears in serpentine, and 

 takes a course parallel to the toj) of the clitf for a few feet and then 

 dies out. This vein is decidedly foliated, the foliation running witli 

 the direction of the vein. Down below, on the beach of Polbarrow 

 itself, between high- and low-water marks, there is another outcrop 

 of what appears to be a continuation of the intrusive dyke seen at 

 the top of the cliff. A gabbro vein about a foot and a half thick 

 runs a course nearly parallel to the beach for 31 feet, and no portion 

 of it exhibits any trace of foliation. Here the onward passage of 

 the gabbro appears to have been barred, and, after an ineffectual 

 attempt to force a passage upwards, the vein turned sharply down 

 at right angles to its former course, and tiiudly broke into a spray of 

 finer veins. Just at this elbow the gabbro is intensely foliated, 

 resembling, if one may use the comparison, streaky bacon. It 

 seems clear that pressure after consolidation can have had nothing 

 to do with the foliation of this rock. Such pressure, had it been 

 applied, must have affected the serpentine as well as the gabbro, 

 but there are no signs of it. Then the pressure that converted a 

 coarse-grained gabbro, in one portion of the vein, into an intensely 

 foliated streakj'-bacon-like mass, ought to have corrugated or streaked 

 the other portion of the vein immediately in contact w4th it, but it 

 has not done so. It ought also to have foliated the boss on the top of 

 the clitf. It is also to be noted that the two veins which are foHated, 

 namely the one at the top of the cliffs and the one at the bottom, run 

 in directions at right angles to each other, the foliation in each case 

 being parallel to the direction of the vein. These facts, which seem 

 inexplicable on the hypothesis of crush or shearing after consolida- 

 tion, seem perfectly natural on the supposition that the foliation was 

 the result of traction or the resistance offered by the serpentine to 

 the passage of the gabbros. 



The evidence summarized above makes it impossible, in our 

 opinion, to explain the foliation in the gabbro as a result of pres- 

 sure-metamorphism subsequent to the solidification of the rock *. 

 The structure .cannot be later than this epoch, and the following 

 hypothesis appears to comply best with all the conditions of the 

 problem. Suppose that the mass at the time of intrusion w^as not at 



* It must be remembered that mirteral banding, which there is no reason to 

 connect with crushing, has already been not seldom observed. It may be noted 

 in the syenite of the Plauen'schengrund, where the usually orientated felspars 

 Ofcasioually form short streaky bands. It is noticed as occurring in certain 

 granites (Hatch, ' Introduction to the Study of Petrology,' p. 83), and in the 

 hornblende-picrite of Penarfynydd (Alfred Harker, Quart. Jouru. Geol. Soc. 

 vol. xliv. (1888) p. 457). We have mentioned it in this paper as occurring in 

 terpentine (e. e. peridotites), and have seen it in diorite and other holucrys- 

 "tiiUine rocks. 



Q. J. G. S. No. 187. 2 L 



