﻿Vol. 64.] STRUCTUEE OF THE ST. DAVId's AREA. 369 



(B 2) — The dominant component of the Treginnis Series (B 2) is 

 a slightly gritty rock, of more basic composition than any other 

 member of the Pebidian. It is at least 500 feet thick, and possibly 

 much thicker in some places; but this horizon is usually so folded, 

 and also distorted by basic intrusions, that an exact estimate is 

 difficult. 



The commonest phase of this subdivision has a characteristic 

 mottled appearance, consisting of dark-green pebbles in a scanty red 

 matrix, thus giving the effect of a red network on a green ground. 

 This red matrix is highly ferruginous, and mostly opaque under the 

 microscope ; but it contains quartz-grains, small chips of lava, and 

 patches of chlorite. The green pebbles consist of highly-vesicular 

 glassy trachyte, the cells of which are filled with epidote and 

 chlorite, a common arrangement being a lining of epidote surround- 

 ing chlorite. Numerous orthoclase-laths occur, commonly measuring 

 from -02 to -03 millimetre in diameter. They are more or less epido- 

 tized, and show marked fluidal arrangement. The vesicles may be 

 so numerous that the rock is almost pumiceous. This mottled type, 

 which weathers in a rusty ferruginous way, covers fully five-sixths 

 of the area occupied by B 2. The matrix, however, may increase 

 in quantity and vary considerably in colour. The most important 

 variation is that which occurs plentifully in the extreme south- 

 west of the district, near Penmaenmelyn and Forth Henllys, in 

 which the matrix makes up at least half of the rock and is dark 

 green in colour. It can be seen in several places, notably on the 

 promontory east of Pen-dal-aderyn, to pass into the common type. 

 This green matrix is a little paler than the red in thin section,, 

 otherwise showing no difference by transmitted light ; but the 

 trachyte-fragments in the only specimen examined are full of ortho- 

 clase-phenocrysts, which I have not found numerous elsewhere. 

 Another variety, with a copious pale-green or pinkish-green matrix,, 

 may be seen near Trefeithan. 



Most of the specimens of B 2 contain fragments of a rock com- 

 posed of abundant augite-phenocrysts set in a hyalopilitic ground- 

 mass of decayed felspars (andesine), augite, iron-ore, and various 

 products of alteration ; and also fragments of a rhyolite with 

 perfect quartz-bipyramids. Both of these lava-fragments contain 

 veins and occasional vesicles filled (before denudation) with — in the 

 augite- andesite, chlorite — and in the rhyolite, calcite. 



The filling of these veins before deposition in B 2 shows that the 

 andesite and rhyolite were already somewhat altered ; but the 

 trachyte-fragments, on the other hand, were still comparatively 

 fresh: for their marginal vesicles have been filled with the ferru- 

 ginous matrix of the enclosing tuff, showing that they were still 

 empty at the time of deposition. Some vesicles have been partly 

 filled by ferruginous matter, and subsequently lined with epidote. 



The big scattered pebbles of red rock which characterize the 

 Treginnis Series show under the microscope a dusty devitrified 

 glass with strong flow, numerous pheuocrysts of orthoclase and 

 plagioclase, and small quartz-bipyramids — apparently a rhyolitic 

 structure. The lava has picked up little bits of a trachyte. 



