682 GEOLOGY. 



In England, the old red sandstone occupies the greater part of Herefordshire ; it 

 spreads over wide tracts of Monmouthshire, Shropshire, and Worcestershire ; surrounds 

 the coal-field of the Forest of Dean and the great coal-basin of South Wales ; and forms 

 the lofty mountains of Brecknock, called the Brecon and Caermarthen Fans, or summits, 

 which rise 2500 feet above the sea, and are the highest in our southern districts. It appears 

 largely also in Devon, and hence the system has been called the Devonian ; but the title 

 which so eminently describes its composition is preferable. For a considerable time, it 

 was regarded as a doubtful point, whether the sandstone formation of Devon was identical 

 with that of Herefordshire, from the former presenting shells which are not found in the 

 latter, and the latter containing fossil fish not discoverable in the former. All doubt upon 

 the subject has however been removed, since Mr. Murchison observed, in the Russian 

 deposits of sandstone, the fish of Hereford and the shells of Devon in the same beds. 



An enormous thickness belongs to the system, equal in various localities to the height 

 of Etna above the level of the sea, and probably averaging 10,000 feet. The following 

 triple subdivision of this vast mass has been made by Mr. Conybeare and Dr. Buckland, 

 which equally applies to the Scottish sandstone : 



1. Upper A quartzose conglomerate passing downwards into chocolate, red, and green sandstone and marl. 



2. Central Cornstone and marl (red and green argillaceous spotted marls) with irregular courses of impure concretionary 



limestone, provincially called cornstone, mottled, red, and green, containing remains of fishes. 



3. Lower Tilestones, finely laminated hard reddish or green micaceous or quartzose sandstones, which split into tiles, con- 



taining remains of mollusca and fishes. 



4. Overlying Carboniferous System. 



The section exhibits the triple group which constitutes the old red sandstone, with the 

 subjacent silurian and the overlying carboniferous systems. It will be seen that the 

 lower member of the series, the tilestone division, is by far the smallest. This fissile 

 formation appears at Pont-ar-Lleche, the " Bridge on the Tiles," in Caermarthenshire, 

 occupying lofty heights, where the strata have been extensively quarried ; and junctions 

 with the Upper Silurians are well seen in the gorge of the Teme, and in the vicinity of 

 Ludlow. The provincial cornstones in the central member of the system, consisting of 

 concretions of very impure calcareous matter, vary in diameter from half an inch to 

 three or four inches, and are quarried for the repair of the roads. The " conglomerate," 

 composing the superior division of the series, comprises pink and white pebbles of quartz, 

 from the size of a hazel-nut to that of a cannon ball* and is well developed in great masses 

 on the right bank of the Wye, to the north of Tintern Abbey. The entire system is 

 eminently arenaceous, and hence its name, the term " old " distinguishing it from another 

 series, above the coal measures, called the new red sandstones, and the term " red " 

 referring to the predominant colour of the strata. This colour, which exhibits various 

 shades, from a dark brick-red to a cream*yellow, arises from iron oxide, the different hues 

 depending upon the amount of iron in the rock, and its state of oxidation, though in several 

 places in Scotland the tint seems to be occasioned by copper. The origin of the metallic 

 oxide is usually referred to the play of volcanic agency during the deposition of the sand- 

 stone, the infusion of the mineral matter rendering the waters uninhabitable by organic 

 life. It has been thought a curious corroboration of this fact, that fossil fishes are most 

 abundantly found in the lightest coloured and least impregnated strata* 



The physiognomy of the old red sandstone districts is not so striking as that exhibited 

 by the more ancient systems, but presents bolder developments than are common to the 

 subsequent formations. The scenery consists of extensive undulating plains, where the 



