

= 4h ‘ 
764 STRATIGRAPHICAL GEOLOGY. [Box VI. _ 
§ 2. Local Development. 
Britain.1—Triassic rocks occupy a large area of the low plains in the 
centre of England, ranging thence northwards along the flanks of the 
Carboniferous tracts to Lancaster Bay, and southwards by the head of 
the Bristol Channel to the south-east of Devonshire. They have been 
arranged in the following subdivisions : 
Rheetic . Penarth beds.—Red, green, and grey marls, and “ White Lias,” 
(New Red Marl.—Red and grey shales and marls, with beds 
of rock-salt and gypsum (Lstheria and Foraminifera). 
Lower Keuper Sandstone.—Thinly laminated micaceous sand- 
stones and marls (waterstones), passing downwards into white, 
brown, or reddish sandstones, with a base of calcareous con- 
glomerate or breccia. 
Middle. . Wanting in England (Muschelkalk of Germany). 
he Mottled Sandstone.—Soft bright-red and variegated 
\ 
Upper Trias 
or Keuper. 
sandstones, without pebbles. 
Pebble-beds.—Harder reddish-brown sandstones with quartzose 
pebbles, passing into conglomerate; with a base of calcareous 
breccia. 
Lower Mottled Sandstone. — Soft bright-red and variegated 
sandstone, without pebbles. . 
Lower Trias 
or Bunter. 
Like the Permian red rocks below, the sandstones and marls of the 
Triassic series are almost barren of organic remains. [Extraordinary 
differences in the development of their several members occur, even 
within the limited area of England, as may be seen from the subjoined 
table, which shows the variations in thickness from north-west to south- 
east : 





| Lancashire | Lele 
| and W. Staffordshire. Warwick 
Cheshire. arwick- 
shire. 
Feet. id tliat: > Repel al 
atime Red marl : all 3000 800 700 
ig 8 Lower Keuper sandstone | 450 | 200 150 
Upper mottled sandstone — 500 | 50-200 absent 
Bunter. Pebble beds . ; . 500-750 = 100-300 | 0-100 
Lower mottled sandstone 200-500 0-100 | absent 




a A arrears | 
Hence we observe that, while towards the north-west the Triassic 
rocks attain a maximum depth of 5200 feet, they rapidly come down to 
a fifth or a sixth of that thickness as they pass towards the south-east. 
South-westwards, however, they swell out in Devon and Somerset to 
probably not less than 2500 or 3000 feet.?, Recent borings in the south- 
eastern counties show that the Triassic rocks are there absent altogether. 
It is evident that the source of supply of the sediment lay towards the 
north or north-west. ‘This is further borne out by the character of the 
pebble-beds. ‘These are coarsest towards the north, and, besides local 
1 See E. Hull, “Permian and Triassic Rocks of Eugland,” Geological Survey 
Memoirs, 1869; H. B. Woodward, Geol. Mag.; 1874, p. 385; Ussher, Q. J. Geol. Soe. 
xxxii. 367; xxxiv. 459; Etheridge, Op. cit. xxvi. 174; A. Irving, Geol, Mag. 1874, 
p. 814; 1877, p. 309; W. 'T. Aveline, Op, cit. 1877, p. 380. 
2 Ussher, Q. J. Geol. Soc, xxxii. 392. 
