224 



Outside the St. David's area we find many other exposures of basic 

 igneous rock in Pembrokeshire. These are included under the general 

 colour for " greenstone " on the geological map, but it must be remembered 

 that this colour has also been extended over areas occupied by rocks of 

 intermediate and even acid composition. 



A considerable exposure of diabase essentially similar to that associated 

 with the Lower Palaeozoic rocks of North Wales occurs in the neighbourhood 

 of Fishguard. It is well exposed near the village of Llanwnda. The dominant 

 rock is a medium grained ophitic diabase (no olivine), with much chlorite 

 and epidote. The chlorite occurs in fine-grained, irregular and spherulitic 

 aggregates, and often contains detached grains or granular aggregates of 

 epidote. Irregular patches of a coarse-grained rock containing long, narrow- 

 bladed crystals of augite occur in the normal ophitic diabase. The augite- 

 individuals in the coarse-grained rock show a decided approach to crystal- 

 line form, and they are almost invariably twinned in accordance with the 

 ordinary law. The felspars are as a rule more or less cloudy in conse- 

 quence of alteration. When examined with a high power the cloudiness 

 is frequently seen to be due to the development of ill-defined flecks and 

 granules of a vividly polarizing mineral. Ragged plates and skeleton 

 crystals of ilmenite are abundant, and are as a rule more or less changed 

 to leucoxene. The diabase above referred to produces marked contact 

 alteration on the surrounding sediments. A magnificent series of basic 

 lava flows occurs in Skomer Island. The successive flows are some- 

 times very thin and highly vesicular. Green earth and calcite occur 

 in the cavities, and veins of epidote and quartz are not unfrequently 

 seen traversing the rocks. Most of the rocks are considerably altered, 

 but some are very fresh. They consist essentially of small lath-shaped 

 felspars, granules of augite and magnetite. Well-characterized serpen- 

 tinous pseudomorphs after olivine occur sparingly in certain varieties. 

 Interstitial matter occurs only in very small quantity. Some varieties are 

 rendered porphyritic by the occurrence of felspar and aggregates of chlorite 

 which may possibly represent augite. The ground-mass of the porphyritic 

 rocks, and the general mass of the non-porphyritic rocks is compact in 

 texture and often shows under the microscope a well-marked fluxion 

 structure. The rocks contain too much felspar and too little olivine to be 

 perfectly typical olivine-basalts ; nevertheless they are evidently basic 

 rocks. The specific gravity of a typical specimen is 2 '87. 



Lake District. Basic igneous rocks have not as yet been described 

 as occurring in any great abundance in the Lake District. In his Memoir 

 on Sheet 101, S.E., Mr. WARD proves that the dominant type of igneous 

 rock in this district is of intermediate composition. He mentions intrusive 

 dolerites (diabases) from only four localities : Wythop Fells, Castle Head 

 near Keswick, Swirral Edge near the summit of Helvellyn, and Longstrath 

 in Borrowdale. None of these are very typical, and all are highly altered. 

 The rock of Castle Head may be conveniently designated a mica-diabase. 

 The ground-mass is composed of turbid plagioclase, irregular plates of a 

 brown, strongly-pleochroic biotite, skeletons of titaniferous iron-ore more 



