212 ME. H. H. THOMAS OX THE [May K)Hr 



of alkaline and subalkaline types. Mineralogically, the more acid 

 members of the series are peculiar in the abundance of soda-rich 

 plagioclase felspars, and in the association of these minerals with 

 porphyritic crystals of olivine,, hypersthene, and augite. The 

 lava-groups and the supposed sills thin out in an easterly direction 

 from Skomer, where the greatest thickness is displayed, a feature 

 which suggests a vent, or series of vents, lying to the west of that 

 island. 



In order of extrusion, except where the series is thinning, there 

 appears to be a sequence from acid to basic and basic to acid. 

 Thus there is a frequent repetition of the same type of rock, but 

 seldom any extreme change in the characters and composition of 

 contiguous flows. 



In conclusion, I wish to take this opportunity of tendering my 

 grateful acknowledgments to Mr. E. T. Howard and Mr. E. W. 

 Small for most liberally placing at my disposal all their microscope- 

 slides and much useful information which they had collected ; to 

 Mr. J. J. Neale, J.P., of Cardiff, the lessee of Skomer, for the many 

 kindnesses which he showed me during my residence on the island; 

 and to the Elder Brethren of Trinity House and Captain Mayar for 

 allowing me to proceed to the Smalls Kocks on board the S.S. Siren. 

 I am also much indebted to Major E. Howell and Mr. T. Picton, 

 who accompanied me to Skomer ; and to Prof. 0. T. Jones, who 

 worked with me over the difficult sections of the AVooltack 

 peninsula. 



EXPLANATION OF PLATE XL 



Geological map of Skomer Island and the adjacent Pembrokeshire coast, 

 on the scale of G inches to the mile = l : 10,500. 



DlSCTJSSIOX. 



Mr. J. E. N". Geeen welcomed this paper as explaining a 

 difficulty. On Ramsey Island, north of Skomer, certain rocks had 

 been mapped, and repeatedly described as pre-Cambrian and basal 

 Cambrian. It was now clear that these were simply the Skomer 

 Series, the 'conglomerate ' (of which the speaker showed specimens) 

 being a nodular rhyolite. This discovery greatly simplified the 

 geology of Ramsey, and enabled the speaker to confirm the Author's 

 views as to the age of the Skomer rocks — since in Eamsey Island 

 they rested, with apparent conformity, on shales belonging to the 

 zone of Didymograptus bifidus. 



Dr. J. S. Elett remarked on the interest of the paper as con- 

 firming the work of previous investigators, and describing types of 

 igneous rocks not hitherto recognized. 



In his work on the Lower Silurian volcanic rocks of the Southern 

 Uplands of Scotland, Dr. Teall had shown that a series of diabases, 

 diabase - porphyrites, pillow-lavas (rich in soda - felspar), kerato- 

 phyres, and soda-felsites occurred — an association very similar 

 to that described by the Author from Skomer, and belonging in 



