196 MR. H. H. THOMAS ON THE [May I9II, 



(d) The Skomerites. 



A very prevalent rock-type on Skomer and the mainland is, as far 

 as I am aware, peculiar to the district, and the name skomer ite 

 has therefore been applied to it. 1 The skomerites are compact, 

 fine-grained, dark-grey rocks, with a tinge of green. They have a 

 finely crystalline appearance, and show minute laths of felspar to 

 the naked eye. They weather with a pale crust io a few milli- 

 metres in thickness, and are not so highly vesicular as the kerato- 

 phyres. 



Specimens collected from a flow beneath the M ewstone quartzites 

 (E 7056) and from above the soda-trachyte of Pigstone Bay (E 7090) 

 may be regarded as typical. These rocks consist of augitc, olivine, 

 albite-oligoclase felspars, accessory iron-ores, and secondary chlorite 

 presumably replacing a tine- grained or glassy base. 



The augite is a greenish variety, and makes a fifth to a quarter of 

 the rock. It occasionally occurs in clots, but usually is evenly 

 distributed as subidiomorphic crystals, grains, and subophitic 

 patches, ranging up to 10 mm. in greatest dimension, but generally 

 much smaller (fig. 9, A, p. 197). There are seldom more than two 

 of the larger crystals in the area of any slide. 



The subidiomorphic crystals and granules appear to belong 

 to the same period of generation, and there was evidently but a 

 very small time-interval between the consolidation of the various 

 constituents. A few of the larger augites show slight resorption- 

 borders, and many of the crystals are twinned, a feature which 

 contrasts them with the augites of the oKvine-basalts (p. 204). 



Olivine is not an abundant constituent, but is usually present 

 building small idiomorphic crystals about 0*3 mm. in length, 

 pseudomorphous in dark green serpentine or strongly birefringent 

 chlorite or iddingsite (?). 



The porphyritic felspars are much decomposed and replaced by 

 chlorite. They are generally not very well formed, and have 

 their angles somewhat rounded. Their refractive indices are lower 

 than that of balsam, and from the wide extinctions they would 

 seem to be nearer to albite than to oligoclase. The felspars of the 

 ground-mass are short ill-defined laths oriented in all directions, 

 with little or no trace of fluidal structure. They have low refrac- 

 tive indices but small extinctions, and thus appear slightly more 

 basic than the phenocrysts ; the}' are still, however, on the albite 

 side of oligoclase. 



Iron-ores are scattered through the rocks as fine dust, and as 

 narrow plates : their quantity is very variable ; and, judging from 

 the relative abundance of secondary sphene, they are titaniferous. A 

 few vesicles may be detected, and these are almost invariably filled 

 with a mass of well- crystallized, secondary, water-clear albite, 

 a little secondary quartz, and chlorite. Veins of these materials 



1 This type is that which was described by Rutley (1881) p. 41 1, Slide No. 3, 

 as basalt or andesite. 



