OLD RED CONGLOMERATE AND SANDSTONE IN HEREFORDSHIRE, ETC. 173 



as laid open upon the sides of the inclined plane by which the limestone is conveyed to the Aber- 

 gavenny Canal. The sides of this inclined plane expose one of the best sections I am acquainted 

 with, extending upwards from the cornstone which appears in the valley, to the conglomerate, the 

 great mass of which lying above the top of the inclined plane, is covered by the mountain lime- 

 stone and millstone grit. The upper masses of the mountains between the Usk and the Wye, oc- 

 cupying various lofty eminences from 1800 to 2545 feet above the sea, and known under the general 

 name of the Black Mountain, together with the Skirrid, are composed of brownish red and greenish 

 sandstones, passing downwards into the concretionary and fragmentary rocks of the next division. 

 The Skirrid is partially capped by quartzose conglomerate, and then exhibits in descending order — 



1. Chocolate red-, and green-coloured thin-bedded hard sandstones. 



2. Do., very finely grained, thick-bedded. 



3. Mottled marls, red and green. 



4. Thick beds of brownish or chocolate red, surfaces marked by irregular blotches or concretions of green and red marl of 



various forms. (See p. 179.) 



5. Marls like No. 3. 



6. Thick-bedded brown sandstone, becoming slightly calcareous and passing down into cornstone. 



The greenish variety of sandstone in this section is composed of grains of sand and quartz, with 

 spangles of mica, in a matrix of decomposed felspar, the colouring matter being probably chlorite, 

 which together with the felspar of the matrix gives a trappean aspect to the rock when viewed in 

 hand specimens. Again, some of the hardest sandstones, particularly where they pass into the 

 blotchy marls and cornstones, are themselves slightly calcareous. Such characters may be observed 

 in many other sections ; but the greenish felspathic rock is a variety never seen in the system of 

 New Red Sandstone. It again occurs in a remarkably compact state in the Old Red Sandstone of 

 Caermarthenshire under Nelson's monument, on the left bank of the river Towey. 



In the Black Forest, the masses dip slightly towards the South Welsh coal-field, the 

 upper strata occupying the summits of long tabular ridges, which rise towards the north- 

 west and present bold escarpments to the valley of the Wye. In some of these ridges, 

 where the lower beds of the zone pass into the inferior cornstone and marl, tiles are 

 largely worked, and other beds contain thin layers filled with fragments of carbonized 

 vegetable matter • but I have never yet detected any remains sufficiently perfect to be 

 compared with known fossil plants. These imperfect fragments of vegetables being ac- 

 companied by thin films of carbonaceous matter, have induced ill-advised speculators to 

 drive galleries into the mountain sides in search of coal. It is scarcely necessary to add, 

 that all these enterprises have failed. Remains of some of the works near the summit 

 of the cornstone group are visible on the eastern side of a steep ravine above the village 

 of Cusop, two miles south of the town of Hay. 



No rock described in this work puts on a more ancient aspect than these sandstones 

 as seen upon the 'north side of the Sugar Loaf, and in the wild gorges by which the 

 great mass of the Black Forest is fissured. We must not, however, judge of the an- 

 tiquity of rocks by their mineral aspect, nor even by their lithological structure ; for, as 

 I shall have occasion to show, there are many portions of the Old Red Sandstone (par- 

 ticularly in Pembrokeshire,) undistinguishable in these respects from the oldest grey wacke 



Y 



