112 THE OLDER ROCKS AT ST. DAVIDS. 



only the metamorphosed remnants, is, of course, a subject only of 

 speculation. 



I have now to enumerate the specimens collected from this 

 series. 



Brynygarn Quarry. The rock here, about a quarter of a mile 

 S. of the city, is a massive crystalline rock of granitic or coarsely 

 crystalline character, not compact therefore, but granular or 

 phanero -crystalline, — a mixture of pale pink, greenish and 

 colourless constituents. Microscopical examination shows it to 

 consist of Felspars, chiefly Orthoclase, a little banded plagioclase 

 is seen attached to one or two of the others : many are much 

 attacked by decomposition, becoming granulated and only semi- 

 lucent^ others, though somewhat clouded, show vivid colours on 

 revolving prism. Quartz very abundant, irregular masses 

 contiguous to each other, or between felspars, &c., very full of 

 minute fluid-cavities with moveable bubbles, scattered or arranged 

 in strings. Chloritic constituent, probably the remains of 

 hornblende though not preserving its form, is partly dichroic, and 

 remains dark blue under crossed prisms, while part has quite lost 

 its depolarising powers (viridite). In some cases decomposition 

 has gone farther, and it has become mixed with Epidote and a 

 black opaque substance for which we may use the term Opacite. 

 There is no mica. A similar rock is seen on the opposite side of 

 St. David's Valley near Clegyr Foia Road : it seemed not 

 necessary to have it sliced for the purposes of this preliminary 

 examination, nor was the rock near Porch Clais and other places 

 where it presents the typical appearance so treated. In this 

 quarry the Dimetian is pierced, as I take it, by thin green bands of 

 a doleritic rock which has been introduced along divisional planes, 

 whether of original bedding or mere joints I could not here 

 determine. There are two bands In this quarry, the upper one of 

 only a few inches — near the surface of the ground — it thins out 

 entirely on one side, and not only so, but it is highly decomposed, 

 being converted and running down into a mere sandy earth : it 



