154 H. HICKS ON THE DIMETIAN AND 



lie in the line of the bedding of their associated quartz rocks. As 

 changes of equal extent, however, have been recognized as due to 

 metamorphism in such rocks in other areas, I venture to retain 

 them for the present as a special group in the Dimetian series with 

 a probably volcanic origin. Prof. Bonney tells me he considers them 

 to be most like a lava now. Localities : Quarries north of the 

 Church Schools, St. David's ; and Caerbwdy valley, between Trefy- 

 nard and Trepuet, &c. 



2. Fine-grained quartz-felsites, very compact and usually of a 

 greenish-grey colour and containing specks of viridite (Appendix, § 2). 

 They are frequently associated with the above-mentioned rocks 

 (No. 1), and interbedded with them. They also extend further 

 south, and may be met with in numerous exposures along the side of 

 the hill to the east of the Cathedral. They were also the chic£ 

 rocks passed through in digging a well, 40 feet deep, at Glasfryri , 

 near the centre of the ridge east of St. David's, a few years since,, 

 but interstratified here with beds of the next series. 



3. Ashy shale-like rocks of a dull green or bluish colour, some- 

 times highly indurated, but usually showing lines of lamination,, 

 They are chiefly associated with the beds No. 2 and the next series ; 

 and they may be seen in one of the paths in the church-yard east of 

 the Cathedral, in some road exposures along Goat Street, and in a 

 quarry on the west side of the St.-David's valley south of the camp. 

 This quarry is about half a mile south of the Cathedral, and is well 

 worth examining. These rocks have also generally been met with 

 in digging most of the wells at St. -David's ; and the chief springs 

 usually occur in them. On microscopical examination these bands 

 prove to be all altered basaltic rocks (see Appendix, § 3). 



4. Compact granitic-looking (granitoid) rocks, usually of a light 

 green, or grey tint, but with sometimes a pinkish hue. The quartz 

 is only partly crystallized, quartz grains occurring usually in abund- 

 ance, and making up in most cases by far the largest proportion of 

 the mass. The felspar is chiefly orthoclase ; and there is also a green 

 mineral, occasionally in considerable quantity. (See Appendix, 

 § 4, and also note at foot by Prof. Bonney.) 



These rocks attain a great thickness, and extend all along from 

 directly south of the city to Porthclaish Harbour, and may be ex- 

 amined in numerous exposures along both sides of the valley ; they 

 are interstratified, however, with a few bands of the green ashy 

 shales, also by quartziferous breccias. 



5. Quartziferous breccias, consisting chiefly of angular and rounded 

 bits of quartz and fragments of rocks like No. 3. (Appendix, § 5.) 



6. Quartz schists (granitoid rocks) of a greenish and pinkish tint, 

 containing frequently some dolomitic and chloritic materials. (See 

 Mr. Davies's notes in my former paper, Q. J. G. S. vol. xxxiii. p. 231.) 



7. Quartzites, usually of a slightly greenish tint, from a small 

 admixture of a chloritic mineral *. 



* The series 4, 5, 6, 7, make up the bulk of the Dimetian, and collectively 

 show that the thickness claimed by me for this formation in my former paper, 

 Yiz. 15,000 feet, is not an overestimate. The beds strike everywhere across 





