THE OLD RED SANDSTONE. 17 
It will be found, however, that this hitherto neglected sys- 
tem yields in importance to none of the others, whether we 
take into account its amazing depth, the great extent to 
which it is developed both at home and abroad, the interest- 
ing links which it furnishes in the zoological scale, or the 
vast period of time which it represents. There are localities 
in which the depth of the Old Red Sandstone fully equals 
the elevation of Mount /<tna over the level of the sea, and 
in which it contains three distinct groups of organic remains, 
Old Red Sandstone is of enormous thickness in Herefordshire, 
Worcestershire, Shropshire, and South Wales, where it is seen to 
crop out beneath the Coal Measures, and to repose on the Silurian 
Rocks. In that region, its thickness has been estimated by Mr. 
Murchison at no less than ten thousand feet. It consists there of — 
“1st. A quartzose conglomerate, passing downwards into chocolate- 
red and green sandstone and marl. 
«62d. Cornstone and marl, (red and green argillaceous spotted marls, 
with irregular courses of impure concretionary limestone, provin- 
cially called Cornstone, mottled red and green; remains of fishes.) 
“‘3d. Tilestone, (finely laminated hard reddish or green micaceous 
or quartzose sandstones, which split into tiles; remains of mollusca 
and fishes.) 
‘‘T have already observed that fossils are rare in marls and sand- 
stonés in which the red oxide of iron prevails. In the Cornstone, 
however, of the counties above mentioned, fishes of the genera Ceph- 
alaspis and Onchus have been discovered. In the Tilestone, also, 
Ichthyodorulites of the genus Onchus have been obtained, and a 
species of Dipterus, with mollusca of the genera Avicula, Arca, Cucul- 
lea, Terebratula, Lingula, Turbo, Trochus, Turritella, Bellerophon, 
Orthoceras, and others. 
“« By consulting geological maps, the reader will perceive that, from 
Wales to the north of Scotland, the Old Red Sandstone appears in 
patches, and often in large tracts. Many fishes have been found in it 
at Caithness, and various organic remains in the northern part of 
Q* 
