UPPER FORMATION. — QUARTZOSE CONGLOMERATE AND SANDSTONE. 171 



20. 



a. Millstone Grit. b. and c. Carboniferous Limestone and Shale (overlying rocks described in the preceding 

 chapters.) 



The Old Red System consists of d, e, f, and g: viz. d and e. Conglomerate and Sandstone; f. Cornstones and 

 Marl; g. Tilestone. 



The Ludlow Rocks (upper formation of the Silurian System) appear rising from beneath the Old Red System at h. 



In the descriptions which follow it will, however, be seen that the distinctions implied 

 by these names are not absolutely peculiar to any one of the three divisions, but are 

 under certain limitations repeated in each. Thus, though tiles are largely quarried in 

 the lowest strata throughout extensive tracts of country, beds almost equally fissile 

 occur partially both in the bottom part of the uppermost, and in the central or corn- 

 stone division: while, on the contrary, at Tort worth and Thornbury in Gloucestershire, 

 and in parts of Monmouthshire, conglomerates are met with below the upper zone; and 

 in Pembrokeshire they even take the place of tilestones in the lowest strata. (See PI. 36. 

 fig. 17. and PI. 35. fig. 6.) 



Let us commence the description of these deposits at the north-western edge of the 

 coal basin of South Wales, where the upper zone or formation is very largely and clearly 

 developed. 



Quartzoss Conglomerate and Sandstone. (See PL 31. fig. 1. and d, e of wood-cut.) 



The loftiest points occupied by the Old Red Sandstone of England are the fans of 

 Caermarthen and Brecon, the former 2590 and the latter 2500 feet above the level of 

 the sea. The southern face of these mountains, like most other parts of the edge of 

 the South Welsh coal basin, slopes in a gentle talus, inclined from ten to twelve degrees 

 towards the centre of the coal-field, and is usually covered by turf or bogs. When 

 these coverings are removed, a conglomerate, composed of pebbles of white quartz in a 

 red matrix, forms the upper member of the Old Red Sandstone, or substratum of the 

 carboniferous limestone. To the south of Abergavenny, and all along the northern 

 edge of the coal-field, the conglomerate occupies this relative position, clipping beneath 

 the limestone and cropping out near the summit of the steep escarpments. In the 

 Blorenge mountain near Abergavenny, the upper beds immediately underlying the lime- 

 stone, are quartzose grits, with a slightly calcareous cement, containing occasionally 

 pink and white pebbles of quartz, varying from the size of a mustard-seed to that of 

 two and three inches, sometimes mixed up with grains of a green colour and compact 

 felspar. Other and lower beds, however, are pure quartzose conglomerates. When 

 fresh quarried, the conglomerate is sometimes of a pink or reddish colour, but after 



