208 ME. H. H. THOMAS ON THE [^ a } r I9II, 



between Martin's Haven and Hopgang. A beautiful rhyolitic con- 

 glomerate is exposed on the shore in the west side of Martin's 

 Haven. I have been unable to prove the existence of grits in the 

 Garland Channel (Skomer), as suggested by Mr. Small. All the 

 half-tide rocks examined were of rhyolitic character ; but some 

 were well bedded, and presented the appearance of sediments when 

 looked at from the top of the cliff. 



Red clays on other horizons are frequent, and occur in Pigstone 

 Bay, below the rhyolite of Tom's House; at South Castle; a little 

 way below the red rhyolite of the Neck and Midland Island ; in 

 Mouse's Haven ; and in association with the dolerites of the Anvil. 



All these rocks were undoubtedly deposited by water, and, 

 although the red clays, which range from 1 to 7 feet in thickness, 

 are similar to the boles of other districts and are undoubtedly due 

 to the weathering of basic lavas, they have not been found in place 

 except in a few instances, but have been deposited by water, as is 

 shown by the common occurrence of spangles of mica. The upper 

 portions of the flows on which they rest occasionally appear to be 

 roughly conglomeratic, the igneous fragments being deeply stained 

 and interbedded in a red clay matrix (as, for instance, Pigstone 

 Bay, the south side of Midland Island, and the upper dolerite of 

 the Anvil). The red clays present the most conclusive evidence 

 of the subaerial character of the igneous rocks, and for the most 

 part are associated with the more basic members of the Volcanic 

 Series. Even when the clays are absent, many of the lava-surfaces 

 show a conspicuous red staining, such as is noticeable on the south 

 side of Grassholm, and on the dip-slopes of Skomer Head, the Neck, 

 and Wooltack Head. 



Examined microscopically, the rhyolitic breccias of the main 

 sediments of Skomer Island might be described as arkose, almost 

 all the fragments, which measure as much as 7 inches across, being 

 of igneous origin. These fragments consist of pieces of soda- 

 rhyolite, broken spherulites, keratophyres, soda-trachytes, and other 

 Skomer rocks, in a fine felspathic matrix consisting of quartz and 

 broken crystals of plagioclase. 



The finer sediments range from felspathic grits to quartzites. 

 They are made up of subangular quartz, angular soda-rich felspar, 

 and a few fragments of trachytic and other acid volcanic rocks 

 set in a more or less silicified clastic matrix. 



True pyroclastic rocks, as has been remarked (p. 185), are very 

 poorly represented in the Skomer Series. One of the best series 

 of tuffs is that exposed in Pigstone Bay. The rocks are well 

 bedded, reach 8 feet in total thickness, and consist of numerous 

 broken acid- plagioclase crystals, as also fragments of perlitic, 

 spherulitic, and microlitic rhyolites, set in a highly felspathic paste 

 composed largely of felspar fragments and microlites. The small 

 patches of silicified clastic material on the surfaces of certain of 

 the keratophyres have already been mentioned (p. 185). 



On the mainland tuffs of intermediate character and several feet 

 thick have been noted beneath the quartzites of Cable Bay in the 

 cliffs of Wooltack. 



