108 ./. E. W. Modes— The Picrite of Foel hvyd. 



Biological Station he had taken T. trimcatus in raid-channel, and that 

 it also occurred at Guernsey. 



So much for a quarter of a century's negative evidence, and the 

 testimony of the fauna of the Torbay Raised Beaches to a rather colder 

 temperature. 



VI. — The Picrite of Foel lwyd, Carnarvonsiiirk. 

 By J. E. Wynfield Erodes, B.Sc. 



DURING a short holiday to North Wales in Whit-week, 1910, 

 I was investigating the igneous rocks to the south of Llanfair- 

 fechan, more particularly the intrusive greenstones of the Geological 

 Survey map. Since the publication of this map, over fifty years ago, 

 several districts within it have been investigated in greater detail, 

 especially as to the volcanic rocks, but much remains to be done 

 petrographically. One of these greenstones turned out to be of 

 exceptional interest, being of a type of rock hitherto, I believe, 

 unrecorded in North Wales, so another visit was made to it in 

 August, 1912. 



My attention was drawn to it in the field by its pronounced lusti'e- 

 mottling, and a fuller investigation revealed in it strong affinities to 

 the augite picrites. It occurs on the southern side of the height 

 Foel lwyd, overlooking tlie celebrated Bwlch-y-ddeufaen Pass, which 

 connects Aber with the Conway Valley, and outcrops on a dip-slope, 

 which owing to the steep southerly dip is often precipitous. Owing 

 to the interstratification of grits, shales, and igneous rocks, the side of 

 the hill is terraced, each hard band forming a crag. One of these, 

 the lowest, is composed of the rock in question, and it appears to 

 form a sill-like mass, possibly a phacolite, intruded between the grits 

 and shales overlying the so-called Bala Volcanic Series. It appears 

 to send off veins into these rocks. It dies out horizontally, and at the 

 same time the feature ceases to be traceaV)le. The crag is heavily 

 glaciated, although, owing to extensive weathering, striations are not 

 evident. 



Petrology of the Intrusion, — In hand-specimens the rock is of 

 a medium grain and of a dark-green colour, becoming lighter on 

 weathering; notable in most specimens for its lustre-mottling. 

 Serpentine bulks to the extent of one-half the rock, diallage is 

 abundant, and felspar is usually to be detected ; pyrites occurs locally. 

 Microscopically the rock is seen to contain — 



Serpentine, pale green, fibrous, nearly isotropic, interstitially and 

 as granules up to "OS inch in diameter embedded in augite, 

 apparently pseudomorphing olivine, which must have been the most 

 abundant constituent of the rock. 



Augite, in large pinkish brown prisms and grains up to "16 inch in 

 diameter, with a very pronounced diallage striation, enclosing olivine 

 ])seudomorphs (poecilitic structure) and plagioclase laths (ophitic 

 structure) ; next to serpentine the most abundant constituent. 



Bastite, fairly abundant in the serpentine in flakes up to 01 inch 

 inleno'th. 



