262 



LYELL'S ELEMENTS OF GEOLOGY. 



Old Red Sandstone. 



elapse before a square area of low land, having a diameter of as 

 many leagues, can be gained from the Gulf of Mexico. 



OLD RED SANDSTONE. 



It was stated that the Carboniferous formation was surmount- 

 ed by one called the " New Red Sandstone," and underlaid by 

 another called the Old Red, which last was formerly merged in 

 the Carboniferous system, but is now found to be distinguishable 

 by its fossils. The Old Red Sandstone is of enormous thickness 

 in Herefordshire, Worcestershire, Shropshire, and South Wales, 

 where it is seen to crop out from beneath the Coal-measures and 

 to repose upon the Silurian rocks. In that region its thickness 

 has been estimated by Mr. Murchison at no less than 10,000 

 feet. It consists there of 



1st. A quartzose conglomerate passing downwards into chocolate-red and 

 green sandstone and marl. 



2d. Cornstone and marl (red- and green argillaceous spotted marls, with irre- 

 gular courses of impure concretionary limestone, provincially 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). 



I have already observed that the fossils are rare in marls and 

 sandstones, in which the red oxide of iron prevails ; in the Corn- 

 stone, however, of the counties above-mentioned, fishes of the 

 genera Cephalaspis and Onchus have been discovered.* In the 

 Tilestones also, Icthyodorulites, of the genus Onchus, have been 

 obtained ; and a species of Dipterus, with mollusca of the genera 

 Avicula, Area, Cucullsea, Terebratula, Lingula, Turbo, Trochus, 

 Turritella, Bellerophon, Orthoceras, and others. f 



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 Fifeshire, where it crops out from beneath 

 the Coal formation, and spreads into the adjoining southern half 

 of Forfarshire ; forming, together with trap, the Sidlaw hills 

 and valley of Strathmore. (See section, p. 99.) A large belt 

 of this formation skirts the southern borders of the Grampians, 

 from the sea-coast at Stonehaven and the Frith of Tay to the 

 opposite western coast of the Frith of Clyde. In Forfarshire, 

 where, as in Herefordshire, it is many thousand feet thick, it 



* Murchison's Silurian System, p. 180. t Ibid., p. 183. 



t See Geol. Trans. 2d series, vol. iii. plates 15, 16, 17. 



