172 



OLD RED CONGLOMERATE AND SANDSTONE. 



long exposure it frequently becomes nearly white, in which state it might be mistaken 

 for the coarser beds of the millstone grit, but it is separated from them by the carbo- 

 niferous limestone. The conglomerate beds occupy a thickness of about two hundred 

 feet, and pass down into chocolate brown, and reddish, coarse-grained sandstone, with 

 blotches of red shale and occasionally a very small pebble of quartz. Near the Caer- 

 marthen fans the conglomerate beds are from three to four feet thick, and inclose 

 fragments of felspar, hard schist, and black slate. Similar beds, with the same rela- 

 tions, form a continuous girdle round the coal basin of the Forest of Dean, and the 

 tourist upon the Wye may observe excellent examples of them in the promontory which 

 advances to the west of the picturesque cliffs of Symonds Yat, between Monmouth and 

 Ross. Again, these conglomerates are seen in great masses on the right bank of the 

 Wye, to the north of Tintern Abbey, from whence they range southwards over a con- 

 siderable district of Monmouthshire, extending along the summit of Wentwood, the 

 highest ground between Chepstow and Usk„ (See Map.) 



In the beds immediately below the conglomerate, the pebbles gradually disappear, until the rock 

 becomes a pure sandstone of brownish and occasionally of delicate gosling green and deep red colours : 

 other beds are much spotted with green blotches on a dark red ground. All the escarpments of the 

 lofty mountains of the Brecon and Caermarthenshire fans, and extending thence into the neighbour- 

 hood of Abergavenny, exhibit a like succession of underlying strata of brownish red and greenish 

 grey sandstones, fine-grained, hard, and sometimes thick-bedded, the mica being, for the most part, 

 intimately mixed with the other components of the stone. These sandstones alternate with bands 

 of red and green argillaceous marl. Where roads are contiguous, the sandstone is quarried for 

 troughs, cider-presses, and all building purposes. Some of the lower layers of this division are so 

 thin-bedded as to split into tiles, a character, however, which is not common. Other courses of 

 the sandstone have a strong flaglike tendency, and the fine-grained purplish brown sandstones of 

 the Fan bwlch-y-chwyth, near Tre-castle, are quarried for flagstones, which are transported as far 

 as Llandovery. Other varieties are sufficiently fine-grained to be used as whetstones. These rocks 

 are separable from the underlying groups, not only by their position, but also by containing no 

 beds sufficiently calcareous to be burnt for lime. It must, however, be observed, that in many 

 situations where this subgroup is much expanded, particularly on the northern sides of the Blorenge, 

 the fans of Brecon, and other portions of the margin of the South Welsh coal-field, it contains oc- 

 casionally thin courses, two or three in a mountain side, of mottled, red and green, very impure 

 limestone, which is undistinguishable from some of the least calcareous beds of cornstone in the 

 central part of the system. The bold scar 1 called the Daren, two miles north of Crickhowell, offers 

 a fine vertical section of a portion of this division. We there see a greenish, fine-grained, thick- 

 bedded sandstone (an excellent building-stone), alternating with purple, red, and green, finely 

 laminated, marls, and other thin courses of hard sandstone and fine conglomerate. In the latter 

 I detected the scale of a large fish which has not yet been described by M. Agassiz. Similar beds 

 of red and green impure cornstones, are observable midway in the escarpment of the Blorenge, 



1 The Daren owes its form to a great subsidence of the rock, the fragments occupying a rough talus above 

 the valley of the Usk. I much regret that my own imperfect sketch of this picturesque scene is not worthy 

 of being engraved. 



