Vol. 67.] THE SKOMER VOLCANIC SEEIES. 177 



abruptly to an elevation of 200 feet ; but, on account of the smaller 

 size of Grassholm, the highest levels attained by this island fall some 

 way short of the usual plateau-horizon, while the Smalls rise only 

 a few feet above the level of high-water at spring-tides. All these 

 islands, and that portion of the mainland which will here bo 

 described, present a somewhat irregular coast-line, the form of 

 which may be easily explained after a study of the geological 

 features. The major indentations are due to differential erosion 

 controlled by variations in rock-texture and rock - composition ; 

 while most of the narrow inlets owe their formation to the erosive 

 action of the sea along planes of weakness determined by steeply- 

 dipping faults. 



The cliffs, as a rule, are fairly steep and give excellent sections, 

 especially on the northern, western, and eastern sides ; but, on 

 account of the dip of the rocks being at a moderate angle to the 

 south or south-east, the southern sides of the islands are generally 

 less precipitous and the cliffs tail off to sea-level in a succession of 

 •dip-slopes. 



Skomer Island, in addition to the planing which it has suffered 

 in common with the mainland, has had all the remaining depres- 

 sions filled with loamy sands and gravels of glacial origin, leaving 

 the solid outcrops projecting, as isolated knolls and ridges, from 

 almost flat surroundings. 



There is little doubt that the Volcanic Series has a more or less 

 continuous westerly extension from the mainland to some way 

 beyond the Smalls, for a distinct ridge is indicated on the Admiralty 

 chart. Moreover, between Grassholm Island and the Smalls are 

 numerous rocks, known as the Hats and Barrels, which are just 

 awash at the low water of spring-tides. 



This ridge, however, although it joins all the exposures of the 

 Volcanic Series, is not parallel to the strike of the rocks, but 

 probably owes its direction to a system of north-and-south faults, 

 with westerly downthrows, similar to those observable on the main- 

 land between Muscle wick and Wooltack, which repeatedly displace 

 the outcrop of the volcanic series northwards. 



It can be proved that the volcanic rocks continue to a point some 

 20 miles to the west of Wooltack, and, on the mainland, they can 

 be followed eastwards and south-eastwards for 6 miles to the neigh- 

 bourhood of St. Ishmael's, 1 on Milford Haven. 



Skomer, Midland, and Grassholm Islands were surveyed during 

 the year 1908, and the Wooltack promontory in the following year, 

 when the Smalls rocks were also visited. 



II. PeEVIOUS LlTEEATUllE. 



The previous accounts of the Skomer Volcanic Series are limited 

 to the writings of quite a few authors, and little of importance w 7 as 

 published until Messrs. Howard & Small undertook a study of the 

 geology of the island. 



1 T. C. Cantrill, in 'Summary of Progress for 1903' Mem. Geol. Surv, 1910, 

 p. 21. 



x 2 



