Vol. 67.] SKOMER VOLCANIC SERIES. 21 J 



rocks which give place to the lower basalts, about 220 feet thick. 

 These basic rocks are followed by the main group of intermediate 

 lavas, represented by skomerites, keratophyres, and trachytes. 

 There is then a return to basic conditions, as testified by the 

 great mass of mugearites and basalts which occupies the centre of 

 the island. A glance at the map will suffice to show that there 

 appears to be a regular succession from acid to basic, and then back 

 again from basic to acid rocks in a rhythmic sequence. Important 

 rhyolitic groups occur at four horizons, and in almost every case 

 arc led up to by increasingly acid, and followed by increasingly 

 basic rocks. 



It will be seen, therefore, that where the greatest thickness of 

 igneous rocks can be studied there is a frequent repetition of types 

 without any abrupt change in the character and composition of 

 consecutive flows. These abrupt changes, however, become more 

 and more obvious as the distance from the vent increases, and as 

 thinning and overlap within the series become more pronounced. 



IX. Summary and Conclusions. 



The Skomer Volcanic Series, which takes its name from Skomer 

 Island off the coast of West Pembrokeshire, has a minimum thick- 

 ness of 2900 feet and a lateral extension of over 26 miles from 

 east to west, from the neighbourhood of St. Ishmael's, on Milford 

 Haven, to beyond the Smalls in St. George's Channel. The positive 

 evidence places the age as pre-Upper Llandovery, but the indirect 

 and negative evidence would suggest that the volcanic rocks belong- 

 to the lower part of the Arenig Series. The rocks are for the most 

 part subaerial lava-flows of extreme thinness and great lateral 

 extent. The subsequent intrusive phase is but feebly represented 

 by a few dolerite sills, and possibly by a mass of soda-felsite. 



There is a paucity of true pyroclastic rocks, but sediments in the 

 form of breccias, conglomerates, quartzites, and red clays, with a 

 total thickness of 450 feet, mark a constant horizon near the centre 

 of the series. The red clays which also occur on other horizons, 

 although partaking of the nature of plinthite, are not in most cases 

 formed by the decomposition of the basic rocks in place, but are 

 water-deposited. 



Of the igneous rocks eight types with several variants have been 

 detected, representing rocks wdiich range from soda-rhyolites to 

 olivine-dolerites. Chemically, they embrace types containing silica- 

 percentages as high as SO and probably as low 7 as 4(5. 



In the more acid members of the series the percentage of total 

 alkalies ranges as high as 8*82, but soda is always in excess of 

 potash. The soda-rhyolites, soda-trachytes, and the two new types 

 skomerites and marloe sites belong to a soda- rich series, and 

 present affinities with the alkaline rocks of Pantelleria. The 

 mugearites, basalts, and dolerites belong to the subalkaline class, 

 and may be compared with the plateau-basalts and basic sills of 

 Tertiary age in the West of Scotland. Thus we have here a mixture 



