Vol. 60.] THE LLYN-PADARN DYKES. 377 



enclosing portions of the cryptocrystalline groundmass. The latter 

 is granular, and appears to consist almost entirely of quartz, and 

 possibly some felspar. There are also numerous laths of a greenish, 

 strongly-pleochroic mineral, with slightly-oblique extinction, which 

 may be microliths of hornblende. There is much opaque granular 

 matter, and some chloritic patches occur. The rock is apparently 

 modified slightly by contact with the ' greenstone.' 



The ' greenstone ' in thin section shows abundant laths of felspar, 

 mostly replaced by micaceous aggregates, yet here and there 

 retaining the optical characters of a plagioclase. Augite is fairly 

 abundant, mostly altered to an opaque brownish substance, but 

 occasionally giving bright polarization-colours. There is also a 

 good deal of opaque leucoxene, resulting from the decomposition of 

 titaniferous iron. Calcite and chlorite-eyes are abundant, although 

 there is apparently no secondary quartz. The rock was originally 

 somewhat like the ordinary ophitic type of dyke-rocks in the Uyn- 

 Padarn ridge, the structure having been obliterated by shearing, 

 crushing, and alteration. 



Although it would perhaps be going too far to generalize from this 

 single occurrence of an acid intrusion in the ' greenstone,' yet the 

 fact remains, and there does not appear to be any escape from the 

 conclusion that we have here a pre-Cambrian basic rock. There 

 are certain other highly-sheared and altered ' greenstones ' in the 

 Llyn-Padarn ridge, which may also belong to this group. A rock 

 from the locality marked C on the accompanying sketch-map (p. 376) 

 is so like the one described above, and differs in so marked a degree 

 from the unsheared rock in the neighbouring dyke south of Cwm- 

 y-glo, that it seems certain that it has been subjected to stresses 

 which have not influenced the Cwm-y-glo intrusion. Unfortunately, 

 I have not been able to discover any but outcrop-exposures of this 

 rock ; it is not unlikely, however, that when this district comes to 

 be mapped on the 6-inch scale, additional proofs will be forthcoming 

 that some of these highly-altered basic rocks are older than the 

 main dykes of the Llyn-Padarn ridge. 



III. Petrographical Evidence. 



In considering the petrographical evidence as to the age of the 

 main portion of the Llanberis dykes (excluding those of pre- 

 Cambrian age), I shall now attempt, to show that it is not possible 

 to separate these rocks from the Bala diabases upon general mine- 

 ralogical grounds ; and that there are indications that these two 

 groups of rocks were in all probability derived from the same 

 magma-basin. At the same time, it must be remembered that, if 

 this supposition be correct, the low horizon at which the Llanberis 

 rocks occur might be expected to disclose certain divergences, 

 resulting from such a differentiation as might take place in the case 

 of the latest phase of an eruptive sequence. Although this point is 

 of the greatest interest, in view of the differentiation-theory put 

 forward by Mr. Harker to account for the sequence of the Bain 



Q.J.G.S. No. 239. 2 c 



