Vol. 55.] SERPENTINE AND ASSOCIATED RO^KS IN ANGLESEY. 297 



times along, but more frequently across the structure-planes. The 

 tufts may be as much as 15 inch (4 mm.) long ; the mineral is 

 pleochroic (green with the long axis parallel to the vibration- 

 plane of the nicol, and very faintly yellowish when transverse), the 

 maximum extinction-angle obtained being 14°. 



We cannot doubt that the rocks here have been greatly crushed ; 

 'Veins or mineral streaks are crumpled ; oblique lines cross the bands, 

 which themselves are probably bounded by shear-planes ; and under 

 the large gabbro-mass to the west is a crushed, almost ' papery rock/ 

 probably representing the soft chlorite-rock described below. To 

 these rocks with actinolite-tufts, the explanation put forward by one 

 of us doubtless applies, namely, that the secondary actinolite has 

 developed in a crushed material from an original augite or hornblende.-^ 

 Thus a relation exists between these rocks with peculiar actinolite- 

 tufts and actinolitic rocks which have been described from other 

 regions ; and even in Anglesey, as we have indicated, the rocks in 

 which this radial growth has occurred are not identical petro- 

 graphically. 



YI. Chlorite-rock and Talc-schist. 



The macroscopic and microscopic characters of the dull-green 

 chlorite-rock, often containing well-formed magnetite, have already 

 been described by one of us.^ The slight development of the schis- 

 tose structure is probably due to the uniform and soft nature of the 

 rock, which was crushed rather than sheared. It is found in many 

 -places, generally as a narrow outcrop, most extensively at Plas-coch, 

 associated, as has been described, with talc-schist. Here, west of 

 the Rhoscolyn road, a small area (measuring about 150 x 100 yards) 

 forms a kind of plateau ; marsh or low meadow-land bounds it 

 abruptly on the north, west, and south. Most of the rock is a 

 schist or schistose, with lamination or banding well marked, although 

 often irregular, the rock in places becoming patchy. It is frequently 

 greyish-green, with dark green or yellowish streaks (epidotic), or 

 with the latter colour spreading irregularly over the greyish-green 

 ground. The original nature of this rock is difficult to determine : it 

 may have been a gabbro or diabase, but it must be placed at present 

 with the ' green schists.'^ The dip varies somewhat, but is about north 

 (say, N.N.E., 35° to 40°). Thus at the quarry * this schist seems to 

 overlie other rocks. 



The northern face of the quarry (now sloping at a high angle) 



1 T. G. Bonney, Quart. Journ. Geol. Soe. vol. liv (1808) p. 368 & vol. xhx (1893) 

 p. 101. It is possible that the tufted growth in the porphyrite, as contrasted 

 with the confused crowding in the rock probably derived from a pyroxeuite, is 

 •due to a more sparse or more scattered distribution of the original mineral. 



2 Quart. Journ. Geol. Soe. vol. xxxvii (1881) p. 44. On p. 43, for ' left ' of 

 road, read ' right.' 



^ As Prof. Bonney states in more than one place — the ' Griine Schiefer ' may 

 be basic igneous rocks ; see Quart. Journ. Geol. Soe. vol. 1 (1894) p. 284. 



* Situated at the south-western corner of the plateau, immediately east of the 

 ■pool marked on the 6-inch Survey map. 



