Vol. 67.] THE SKOMER VOLCANIC SERIES. 175 



5. The Skomer Volcanic Series (Pembrokeshire). 1 By Herbert 

 Henry Thomas, M.A., B.Sc, F.G.S. (Head January 25th, 

 1911.) 



[Plate XI — Geological Map.] 



Contents. 



Page 



I. Introduction 17.") 



II. Previous Literature 1 77 



III. The Stratigraphical Eelations and Age of the 



Series 179 



IV. The Volcanic Rocks 183 



(a) The various Kock-Types. 



(b) Their Geographical Distribution. 



(c) The Rarity of Tuffs and Agglomerates. 



V. The Petrography of the Volcanic Rocks 1 85 



(a) The Soda-Rhyolites and Felsites. 



(b) The Soda-Trachytes. 



(c) The Keratophyres. 



(d) The Skomerites. 



(e) The Marloesites. 

 ( f) The Mugearites. 



(f/) The Olivme-Basalts. 

 (k) The Olivine-Dolerites. 



VI. The Associated Clastic Rocks 207 



VII. The Chemical and Mineralogical Composition of 



the Lavas 209 



VIII. The Sequence in Time of the various Rock-Types. 210 

 IX. Summary and Conclusions 211 



I. Introduction. 



The region in which this volcanic series is developed occupies an 

 extensive area in the extreme west of Pembrokeshire on the 

 southern side of St. Bride's Bay, and includes the islands of 

 Midland, Skomer, Grassholm, and the Smalls. 



Skomer is by far the largest of the islands, and is situated about 

 two-thirds of a mile from that part of the mainland which is known 

 as Wooltack. It has a length, from east to west, of about 2 miles, 

 a breadth of a little over a mile, and an area of 722 acres. 

 Between it and the mainland lies the small island of Midland, with 

 an area of 21^ acres, which is separated from Skomer by the Little 

 Sound, only about 100 yards in width, and from the mainland by 

 the broader stretch of water known as Jack Sound. 



Grassholm Island lies 8 miles to the west of Skomer and covers 

 an area slightly less than that of Midland, while the Smalls, which 

 occupy only three-quarters of an acre, lie 8 miles to the west of 

 Grassholm. 



Skomer Island is a detached portion of that plateau which is so 

 well marked in the west of Pembrokeshire : it rises more or less 



1 Communicated by permission of the Director of H.M. Geological Survey. 

 Q. J.G.S. No. 260. x 



