5S : T, DAVIES ON THE MICROSCOPIC STRUCTURE 
I think this rock should be grouped with the fine-grained spherulitic 
quartz-felsites of the St. David’s Board Schools. 
No. 31 is a breccia from Treginnis-Uchaf Field, S.W. of St. 
David’s. This is similar to No. 28. Contains large fragments of a 
quartz-felsite in which the felspar crystals are only recognizable 
between crossed nicols ; some of them possess the same cryptocrystal- 
line structure as parts of the ground-mass. There are also frag- 
ments of a felspathic basic rock. The same granular depolarizing 
mineral pervades this breccia. 
No. 32, from a quarry in field on the path to Emlych, N. of St. 
David’s. A greenish rock with a somewhat schistose aspect. This is 
a quartz-felsite with much of a fine-grained black substance 
(opacite ?), irregularly distributed, and sometimes closely aggregated 
in patches, giving the section a brecciated aspect. There is much 
chloritic and some epidotic mineral present. 
No. 33, from Carn-ar-wig, Ramsey Sound, is macroscopically a 
schistose eleva rock with a bright-green nee Under the 
microscope is seen to present a fluxion-structure, enclosing fragments 
of microcrystalline felsites with quartz and felspar crystals, which, 
however, do not, as a rule, lie with their longer axes parallel to 
the apparent foliation. The substance to which the schistosity is 
due is not arranged in continuous bands, but lies in small, wavy, 
lenticular groups. It is a schistose quartz-felsite (old rhyolite). 
No. 34, from east of Pen-y-foel. This is a diabase, rich in olivine 
and felspar, and poor in augite. The felspar is in small columnar 
crystals much altered, and presenting, between crossed nicols, a 
microcrystalline structure. 
No. 35 is from Pen-maen-melyn. ‘This rock has apparently been 
much broken up and decomposed. Its fissures, which are very 
numerous and run in all directions, are filled with a fine crystalline 
carbonate of lime, which gives the whole a brecciated aspect. The 
destruction of the felspar has probably supplied the grey dust which 
- pervades the whole. The augite has been broken up and in places 
partly replaced by a yellowish dichroic mineral. There are sufficient 
traces left, however, of both of these minerals to show that the rock 
was originally a felspathic diabase. 
No. 36 is a dark-grey ashy-looking rock from Carn-ar-wig. It 
appears to be a brecciated ash. It contains fragments of a rock 
resembling a felspathic diabase mixed with crystals and fragments 
of felspars, which have been so altered as to present a fine-grained 
microcrystalline structure between crossed nicols. There are some 
fragments of felsites with a few quartz grains. 
No. 37, from east of Castell, is a dull purplish ashy rock with 
some epidote. In thin section it presents similar characters to 
No. 36, but is of finer materials, and the felspar crystals are 
more abundant. It contains much opacite and the pale-green 
mineral. 
No. 38. West.of Rhosson Rock. Thisisavery compact greenish | 
rock with minute brownish-red spots. In thin section with ordinary 
light there are seen abundance of columnar crystals, which are pro- 
