252 



ME. C. A. MATLEY ON THE 



[May 1900, 



cut from a junction of the basalt with 



Fig. 6. — Faulted basaltic dyke, near the old 

 limestone-quarry on the coast, near 

 Llanbadrig Church. Seep. 251, 



[Width of dyke = about 15 inches.] 



the Llanfechell Grit 

 south of Groes-fechan 

 shows that the basalt 

 has undergone some 

 shearing parallel to 

 the junction; but with 

 these exceptions I have 

 observed no effects 

 of pressure on these 

 rocks. 



These basalts and 

 dolerites are usually 

 fresh-looking black 

 rocks with only a thin 

 weathered crust, are 

 as a rule fine-grained, 

 and sometimes have 

 large phenocrysts of 

 felspar. Some 

 specimens are rich 

 in pyrites, others ex- 

 hibit small spheroidal 

 amygdules filled with 

 calcite and chlorite. Some upstanding bosses of coarse ophitic 

 dolerite are found, in which the augite has been converted to horn- 

 blende. It is uncertain, though not unlikely, that these date from 

 the same period. The Llanbadrig dykes already mentioned weather 

 differently from most of the basalts ; they are externally ferruginous, 

 are more deeply weathered, and below the crust are quite pale in 

 colour. The microscope shows them to be almost tachylytic. 



The following notes, descriptive of the microscopical structure of 

 these rocks, have been kindly supplied by Prof. Watts : — 



[N .A. 112; Llanbadrig : dyke represented in fig. 5, p. 248.] Once 

 a glassy basalt, with large phenocrysts of felspar and smaller ones of 

 felspar and probably augite, all highly decomposed. Altered glassy 

 groundmass full of skeleton-crystals of magnetite. 



[N.A. 113; same locality: dyke represented in fig. 6.] Por- 

 phyritic basalt. Groundmass brown and glassy, full of microliths of 

 magnetite and of long, slender felspar-microliths. 



[N.A. 51 ; between Porth Padrig and the Vicarage,] Porphyritic 

 basalt or andesite. The porphyritic felspars are well shaped, but 

 have undergone a micaceous alteration. The groundmass is mainly 

 made up of felspar-microliths set in cryptocrystalline interstitial 

 matter. 



[N.A. 29 ; boss between Caerau and Nanner.] Coarse, ophitic, 

 hornblendic dolerite, containing augite. 



[N.A. 125; south of Groes-fechan.] A porphyritic basalt with 

 phenocrysts of decomposed felspar and pseudomorphs after olivine. 

 Groundmass very fine-grained and possibly glassy, with magnetite- 

 and felspar-microliths. The rock includes some highly corroded 

 grains, apparently xenoliths, of quartz. 



