Vol. 50.] OF THE NEIGHBOURHOOD OF BTJILTH. 575 



entirely absent, its place being taken by cblorite. There is also a 

 large amount of secondary quartz and calcite. 



In the road-cutting at the back of Pen-cerig House the rock 

 resembles, in haud-specimens, that of Pen-cerig Wood. At the 

 junction of the diabase with the Llandeilo Shale, the latter is, for a 

 space of 3 or 4 inches, converted into a hard, grey, porcellanous 

 rock ; beyond this it does not differ from an ordinary black shale, 



Fig. 2. — Section seen in the road-cutting at the bade of 

 Pen-cerig House. 



&~~^— ^^VAV/*+\V+V+VV+++*++b 



-i. 7^ : — ~- _;. -.__ _ ; _ /v + V/A^AV.VyT+ 



C Level of Road ° 



[Length of section = about 18 yards.] 



a — Llandeilo Shales. b = Diabase. 



c — Junction of shale and diabase, obscured at this spot. 



except in being somewhat more indurated. A section of a junction- 

 specimen shows that the line of division between the two rocks is 

 well denned. The igneous rock has become very fine-grained, 

 consisting of minute felspars and chlorite ; some secondary quartz is 

 present, and a large amount of calcite. The shale next the diabase 

 shows a fine-grained groundmass in which occur what appear to 

 have been cavities, some of which are now filled with quartz, 

 others with a pale-green chloritic mineral, often possessing a radiating 

 fibrous structure. There are also large irregular patches of calcite. 



In Harper's Quarry, 1 \ mile south-west of Pen-cerig House, 

 the diabase is seen on the southern side above the Llandeilo Beds ; 

 the latter are altered into a hard, compact, and well jointed rock, 

 having a black or light grey colour. The igneous rock is rather 

 fine-grained, and consists of the usual minerals, but the augite is 

 now represented by pale green chlorite ; and there is a large amount 

 of a semi-opaque material and calcite. 



The western sill, the smallest of the four, extends from just north 

 of Gwern-y-fed-fach across the Wye to Park Wells. It is exposed 

 near Gwern-y-fed-fach, in the bed of the Wye, in a quarry at Park 

 Wells, and in a road-cutting north of Park Wells, where the 

 junction with the Llandeilo Beds is seen. It is rather a fine- 

 grained greyish rock : a section of a specimen from north-west of 

 Gwern-y-fed-fach shows the plagioclase to be rather less abundant 

 than usual, but very much decomposed ; the augite is clear and 

 colourless, and occurs in ophitic plates extending over large areas 

 and enclosing numerous felspars ; magnetite occurs in irregular 



1 The fossils found in the Llandeilo Beds here are remarkably well preserved, 

 and include, amongst others, Ogygia buchi (Brongn.), Trinucleus Jimbriatus, 

 Murch., Ampyx nudus (Murch.), and Siphonotreta micula, M'Coy. 



