ON THE CRYSTALLINE ROCKS OF THE LIZARD DISTRICT. 403 



as already stated, also occurs on that coast. One or two intnisions 

 of the same rock are found on the east coast. Por instance, there 

 is a well-marked vein in a cove north of Pen Voose *, and another 

 (hadly exposed and so not (juito certain) south of the same, at the 

 farther end of the beach. Dykes of a porphyritic diabase are occa- 

 sionally found cuttinj^ the Ilornblendic Group, as, for example, near 

 Polpeor (where it has been already described), on Carnbarrow, near 

 Ogo-dour Cove, and at the headland to the south, where thei'c are 

 two dykss, one fine-grained, the other (and later) porphyritic with a 

 compact base. This very interesting section has been described by 

 Mr. Fox, with notes by Mr. Teall t. The granite veins which we 

 have noticed above in the Hornblendic Group are more probably ap- 

 proximately synchronous with those in the Granulitic Group, and 

 so are anterior to the date of the serpentine. The last rock and the 

 gabbro are repeatedly cut by basic dykes, many of which have been 

 noticed by previous writers. Of these there are numerous varieties ; 

 most of them are now hornblendic rocks, but it is probable that 

 many, if not all, were originally augitic. In a few cases, as once 

 or twice in Kennack Cove, they are practically indistinguishable 

 from the dioritic members of the Granulitic Group, and we have to 

 rely upon field evidence, but in many no such difficulty exists. Oc- 

 casionally they exhibit an approach to foliation, as in the well-known, 

 dyke on the south side of Caerleon Cove, the exterior of which 

 exhibits a slightly foliated structure ; this we now consider, after 

 careful re-examination, to be due to difl^rential movements during 

 solidification, not to subsequent pressure, for the mode in which the 

 structure occurs agrees better with the former hypothesis, and there 

 is nothing to support the latter. The dykes in the serpentine at 

 Coverack, and in the Crousa-Down gabbro, have already received 

 some notice, but one or two additional details concerning the latter 

 may be worth adding. As already stated, it is frequently traversed 

 by dykes of a basic rock, which varies from compact to granular, 

 and is sometimes slightly porphyritic. Of these dykes, however, we 

 have not thought it necessary to examine more than about a dozen 

 specimens in all, four or five of which are from the gabbro. The 

 former have been magma-basalts, and sometimes at the edges almost 

 tachylytes. They are more or less altered, but numerous lath-like 

 crystallites, with occasional small scattered crystals, of plagioclase 

 can still be recognized, and small oval or oblong greenish patches, 

 occupied by an aggregate of flakes giving bright colours with the 

 crossed nicols, probably indicative of the former presence of larger 

 grains of an augitic mineral. Others, fine-grained but holocrys- 

 talline, may be classed as epidiorites ; one at least is still an ophitic 

 dolerite. 



But the gabbro is also cut by another rock, which, though similar, 



* This was discovered by Messrs. Fox and Teall ; we have not seen it, for 

 the rock-face is only visible from the sea, but tbey secured piiotograpbs which we 

 have examined. This is an interesting case, because the granite cuts the gabbro, 

 as it was said to do by De la Becbe, a fact of which Prof. Bonney did not succeed 

 iu finding a proof (Quart. Journ. Geol. Soc. vol. xxxiii. (1877) p. 915). 



t Trans. Roy. Geol. Soc. Cornwall, vol. xi. pt. iv. (1890) p. 213. 



