410 C. A. Raisin — The Serpentines of the Lleyn. 



traces of ophitic structure, some of the hornblende being bluisb-green 

 and very dichroic. In one brecciated example, granular epidote is 

 scattered abundantly over the slide, replacing crystals and forming 

 veins. 



3. TyJien? — A specimen better preserved apparently than those 

 examined by Mr. Elsden consists of fairly clear felspar and augite 

 with an intimate ophitic structure, and contains some viridite patches 

 marking probably another pyroxenic or allied mineral. Ilmenite 

 and pyrite are present. A much more compact rock appears to be 

 enclosed within this fine dolerite, probably inti-uding into it, as at 

 Trefgraig and Methlan. 



4. Methlan.'^ — A diabase or ophitic dolerite with some alteration 

 products. A compact dyke (1^" wide) is exposed by the side of 

 the road. 



Of the next four masses, two are described by Mr. Barker as 

 including decomposed basalts.^ The Careg Fawr crag is com- 

 paratively small and uniform, but at Forth din lleyn, as has been 

 shewn, and also at Careg, a more complex mass occurs. Some 

 examples from the latter place, like a specimen fi-om near Hendre 

 uchaf, show more pronounced microlithic development, but at all 

 the localities we find a compact rock now altered, doubtless once 

 glassy and of basic or fairly basic composition. 



5. Forth din lleyn. — We are indebted to Professor Bonney for 

 the description of these agglomerates and diabases, and for an 

 examination which proved the absence of serpentine in this part of 

 the Lleyn.* He described the rocks on the eastern coast of the 

 promontory, and noted on the western side a similar diabase cut by 

 amygdaloidal doleritic dykes, doubtless, as suggested, of later date.^ 

 Certain varieties are porphyritic, containing brighter green felspars 

 within a dull green matrix. The ground mass in two rocks, ^ 

 although they are not entirely identical, shows under the microscope 

 many of the characters which occurred in certain specimens 

 described by Mr. Elsden.' In one slide, epidote not only replaces 

 crystals, but small rounded amygdules consist partly of that mineral, 

 partly of viridite and of calcite. The porphyritic felspars are 

 decomposed and saussuritic, often averaging ^' in length. They 

 form one point of likeness between these rocks and a specimen 

 which I obtained further southward in the Lleyn. The well known 

 Lambay porphyry also bears some resemblance in the character of 



1 Mr. J. V. Elsden, he. cit. p. 304. Eev. J. F. Blake, he. cit. p. 531. Mr. A. 

 Harker, he. cit. p. 87. 



2 Mr. J. V. Elsden, he. cit. p. 304. Mr. A. Harker, he. cit. p. 87. 



3 Mr. A. Harker, loe- cit. pp. 87, 88. 



^ Q.J.G.S., 1881, vol. xxxvii. pp. 48-50. Prof. T. G. Bonuey. 



fi Q.J.G. S., 1881, vol. xxxvii. p. 50. Prof. T. G. Bonuey. Compare also 

 Q.J.G.S. 1888, vol xliv. p. 461. Mr. A. Harker. The Bala Volcanic Series of 

 Carnarvonsliire. Mr. A. Harker, p. Ill ; Geol. Mag. 1880, Vol. VII. p. 457. 

 Mr. A. S. Reid. 



^ One of the rocks was obtained from a boss on the grassy slope and has a bright 

 green polished surface with streaks coloured by red halmatite. Sheep are continually 

 rubbing themselves against the projecting stone, and the polishing is evidently due to 

 their action. . '' Mr. J. V. Elsden, he. cit. p. 308. 



