PRE-CAMBRIAN ROCKS OF ST. DAYID's. 295 



of detail may be observed to be common to the rocks of the two areas 

 as to indicate that volcanic phenomena must have recurred under 

 much thej3ame conditions throughout Palaeozoic time in the British 

 area. 



The visible thickness of the volcanic group in the St. David's district 

 appears to be about eighteen hundred feet ; but as its base is not 

 brought up to the surface, the total amount may be greater. A 

 continuous section of the rocks is exposed on the sea-cliff between 

 Ogfeydd-duon on Eamsey Sound and the east side of Porth-lisky. 

 This section repeats the members of the group on each side of the 

 isocline, the axis of which must cut the coast-line somewhere be- 

 tween Pen-maen-melyn and Pen-y-foel. I had not time to attempt 

 a detailed examination of the successive members of the group as 

 exhibited in this section ; but the main subdivisions appeared to me 

 to be as follows, in descending order : — 



4. Fine tuffs and silky schists (occasional breccias and agglom- 

 erates), seen at Porth-lisky and Nun's Chapel on the east side, and 

 at Ogfeydd-duon on the west side of the fold. At the latter locality 

 only the upper beds are well exposed. 



3. Diabase sheets with intruded quartz porphyry and hardened 

 tuffs, Pen-maen-melyn, Pen-y-foel. The lavas are considerably 

 thicker on the west side. 



2. Compact green granular tuff. Inland from old copper-mine at 

 Pen-maen-melyn, and near Pen-y-foel. 



1. Thick purplish-red green-flecked tuff, with abundant small 

 lapilli of felsite. This conspicuous rock, in many successive and 

 somewhat variable beds, extends nearly the whole way between the 

 headland at Pen-maen-melyn and Pen-y-foel. It dips at high 

 angles towards the N.W., and shows intercalated shaly bands. It 

 must occupy the centre of the fold ; so that the south-eastern dips 

 are inverted ones, and the rocks on that side are a repetition of what 

 is seen on the north-western side of the axis. 



From this merely tentative stratigraphical arrangement it is 

 evident how large a proportion of the whole mass of the volcanic 

 group consists of tuffs. 



Tuffs. — These predominant members of the group present many 

 varieties of colour, from dark purple, through tints of brick-red and 

 lilac, to pale pink, yellow, and creamy white, but not unfrequently 

 assume various shades of dull green. They vary likewise in texture 

 from somewhat coarse breccias or agglomerates, through many grada- 

 tions, into fine silky schists in which the tuffaceous character is almost 

 lost. Generally they are distinctly granular, presenting to the naked 

 eye abundant angular and subangular lapilli, among which broken 

 crystals of a white, somewhat kaolinized, felspar and fragments of 

 fine-grained felsite are often conspicuous. Examination on the 

 ground suggested that the greater part of the tuffs has been derived 

 from the explosion of basic rocks similar in character to the diabases 

 now found associated with them. This appeared to me to be particu- 

 larly the case with the purple, red, and dark-green varieties, which 

 constitute so large a proportion of the whole On the other hand, 



z2 



