Vol. 67.] 



SKOMER VOLCANIC SEETES. 



203 



Structurally, and in the mineral contents, the agreement is close, 

 except in the absence of the brown hornblende and mica from the 

 Skomer rocks 



(g) The Olivine-Basalts. 



The basalts are dark compact rocks, vesicular in their upper 

 and lower portions, with a finely crystalline aspect and a purplish 

 tinge (due to the somewhat deeply coloured augite which they 

 contain abundantly). They are all of the granulitic or hypidio- 

 morphic type, and consist essentially of magnetite, olivine, augite, 

 and basic plagioclase. 



A =01i vine-basalt, porphyritic, showing porpbyritie labradorite and corroded 

 and decomposed olivine-phenocrysts ; from the cliff south of Eull Hole 

 (Skomer). Slide E 7480, ordinary light. X 25 diameters. 



B = 01ivine-basalt, non-porphyritic, with well-developed fluxion -structure and 

 granular augite ; from inland crags south of the North Cliff rhyolite 

 (Skomer). Slide E 7102, ordinary light, X 25 diameters. 



This may be divided for purposes of description into two types 

 which merge insensibly one into the other : (a) a porphyritic type, 

 with porphyritic plagioclase and a ground-mass usually devoid of 

 flow-structure (fig. 11, A, above) ; and (b) a non-porphyritic type. 

 with beautifully developed flow-structure (fig. 11, B). 



The porphyritic felspars are all much decomposed, passing into 

 micaceous aggregates and being replaced by chlorite. They appear 

 to be a labradorite of medium composition, and in extreme 

 cases reach 1 cm. in length, but more often measure about 5 mm. 



