TJie Ordovician Lavas of Mynydd Prescelly. 317 



•a certain amount of ferruginous matter — cubes of limouite after 

 pyrites. Flow-banding is apparent in slight differences in the relative 

 proportions of the different constituents, and to a lesser degree in 

 the texture of the rock. 



The darker patches in the rock prove to be fine-grained aggre- 

 gates of felspar microlites in a matrix of chlorite, and this material, 

 which is often somewhat " variolitic " in structure, occurs both 

 as filling in vesicles and as irregular patches. One such patch is 

 shown in Fig. 5, and contains, in addition to its normal constituents, 

 several ctibes of limonite after pyrites and a very large apatite. It 

 is possible that the chloritic matrix represents originally glassy 

 matter, as has been suggested in the case of the similar green material 

 occurring in the rhyolites of Carn Alw.^ 



On the same line of strike a dark grey rock is exposed by a spring 

 high up on the hillside, 500 yards east by south of Pant-maenog. 

 Though very similar in composition to the rock in Craig-y-cwm, 

 this is rather different in structure, the ground -mass being more 

 granular, while the felspar microlites are more ragged and flow- 

 banding is not developed. 



(b) The Soda-Trachytes. 



In the boulder-strewn valley between Cnwc and the lower slopes 

 of " Prescelly Top " the lavas are of a distinctly " trachytic " type. 

 As a rule, they are not so fresh as the more massive and rhyolitic 

 varieties developed round Greenway. In the hand-specimen these 

 rocks are usually greenish in colour, from their contained epidote 

 and chlorite, fine-grained to flinty in texture, and often showing 

 fluxion-banding and vesicular structures. 



One of the freshest — a pale green fine-grained rock from a point 

 500 yards south-east of Pant-maenog — shows under the microscope 

 (Fig. 5) phenocrysts of soda-orthoclase and albite usually about 

 0-7 in. in length and commonly associated together in groups, 

 recalling '" glomero-porphyritic " structure. With these is often a 

 certain amount of epidote occasionally in pseudomoiphs after 

 pyroxene and some interstitial quartz. The ground-mass consists 

 of little laths of felspar with interstitial chlorite and a variable 

 quantity of epidote and granular sphenc ; there is also a little 

 interstitial quartz. There are a number of round vesicles from 

 •05-'25 in. in diameter, which are filled with quartz. 



The felspar microlites are in general about -01 in. in length, and 

 from five to ten times as long as broad. Some are either untwinned, 

 or simj^le tv\'ins giving ahnost straight extinction, and arc a soda- 

 orthoclase. The majority, however, are a plagioclase, and their 

 extinction angles, which range up to about 15°, and low index 

 of refraction indicate a composition near to albite. 



The structure of the ground-mass is " trachytic ", the felspar 

 microlites showing marked fluxional arrangement. In my own 



^ Parkinson, op. cit., p. 468. 



