^^^' 55-] SERPElS'TIlSrE AND ASSOCIATED EOCKS IN ANGLESEY. 295 



secondary origin. They have a considerable resemblance to the 

 radial growth described in actinolite-schists by one of us,^ and a 

 still closer similarity to the tufts in the rocks of Bastogne in the 

 Ardennes.^ 



(a) In the district west of Llyn Penrhyn, we endeavoured to 

 trace the junction of gabbro and serpentine, and obtained one 

 specimen in which a small triangular patch resembled a tongue of 

 gabbro. This, on microscopic examination, was seen to be a coarsely 

 crystalline aggregate, mainly plagioclase-felspar and pyroxene, but 

 the surrounding mass contained acicular tufts. The interpretation 

 of the specimen is not easy, but we have come to the conclusion that 

 the latter part is more probably crushed material of the gabbro, the 

 apparently intrusive tongue being a piece which has escaped ; for 

 other fragments can be detected, and the compact rock is a confused 

 aggregate, partly felspathic, which shows signs of a certain amount 

 of reconstitution.^ Further, in the fragmental remnant of the 

 gabbro, the green constituent (doubtless derived from the pyroxene) 

 seems, as it were, to grow out from the ends of the original crystal 

 in acicular actinolite similar to that of the tufts. Thus the gabbro 

 is evidently brecciated and crushed near the margin of its outcrop, 

 and the radial tufts seem to have been developed in the crushed 

 material. 



(6) On the southern shore at the head of the large inlet south of 

 Penrhyn-Eadog, we pass from a rather large boss of serpentine 

 (quarried in the held), across a small interval, to a soft dark 

 chloritic rock forming a narrow strip in the cliff. Here begins a 

 large mass, mainly of gabbro similar to that of Graig-Dinas. jS'ext 

 to the chlorite-rock, however, a compact dull grey-green rock with a 

 streaky structure (parallel to the margin) reminds us of the Tyddyn- 

 y-cob specimens, previously described by one of us.'^ Under the 

 microscope it presents a certain similarity to these, but is without 

 residual fragments of recognizable gabbro. Tufts, however, Oi 

 actinolite occur, apparently similar in origin to those in the 

 specimen last described, and the same explanation is probably 

 applicable in this instance. 



(c) Near the crushed gabbro west of Llyn Penrhyn two outcrops 

 occur, immediately south of the small streamlet, both being at the 

 northern edge of ' green schists,' which are well shown in a small 

 quarry in the western section, and just appear in the eastern boss. 

 North of both, on the other side of the water, rise masses of the 

 crushed serpentine already noticed. In the quarry the pale greenish 

 slabby rock with a rather gritty texture forms a low double 

 anticlinal, the northern slope of which is in contact with one of the 



^ ' On the Garnet- Actinolite Schists of the St. Gothard Pass,' Quart. Journ. 

 Geol. See. vol. Hv (1898) pp. 357-371. Figs. 2, 3, & 4, pp. 362-383, would 

 represent the actinolite of the Anglesey rocks, as seen under the microscope. 



^ A. Renard, Bull. Mus. Roy. Hist. Nat. Belg. vol. i (1882) ; see especially pi. ii. 



^ That pressure has acted on the rocks of this area is vrell shown in the 

 adjacent serpentine ; see p. 277. 



■* Quart. Journ. Geol. Soc. vol. xxxvii (1881) p. 41. 



