270 PKOF. E. HULL ON THE GEOLOaiCAL EELATIONS 



stone formation " of Hereford and Monmouth appears from its po- 

 sition to be inevitably the equivalent of the Middle and Lower De- 

 vonian beds, the palseontological differences are remarkably con- 

 trasted. As Sir H. T. De la Beche originally, and Prof. Ramsay 

 subsequently, have so well pointed out, the discoloration of the 

 waters by peroxide of iron seems to have had the effect of driving 

 moUusks and corals from the region of the Devonian rocks north 

 of the Severn, while fishes (such as those whose remains are found 

 in the Cornstone group) could readily swim without injury in the 

 waters from which the peroxide of iron was thrown down, so long 

 as they did not disturb that substance at the bottom*. Hence, 

 while the Devonian estuary over the area lying to the north of the 

 Severn was unfitted to be the habitat of moUusks, corals, and 

 crinoids, these animals flourished abundantly in the more open and 

 purer waters of the sea which ranged over Devonshire, the south of 

 England, and eastward towards the Ehine. The beds containing 

 Serpula, discovered by Mr. Salter at Galdy Island, were probably 

 situated on the margin of the estuary on the one hand, and of the 

 more open sea on the other. It would therefore tend to a clearer 

 appreciation of the true relations of these strata if they were to be 

 called by some term such as that of " lacustrine Devonian beds " 

 rather than that of " Old Eed Sandstone," to which they have cer- 

 tcdnly no titlef, 



XII. Scotland. 



According to the latest researches on the subject of the Old Eed 

 Sandstone of Scotland, there are but two divisions — a "Lower," 

 passing down conformably into the Upper Silurian shales, and an 

 " Upper," graduating upwards into the Lower Carboniferous Sand- 

 stones, with a complete discordance between the two series J. These 

 relations correspond exactly to those of the Glengariif beds of the 

 south of Ireland on the one hand, and of the Old Eed Sandstone on 

 the other. 



" The Lower Old Eed Sandstone "of Scotland consists of sandstones, 

 shales, and conglomerates, with cephalaspid and pteraspid fishes and 

 large eurypterid crustaceans. '' The Upper Old Eed Sandstone," 

 consists of red a,nd yellow sandstones with a base of conglomerate. 



* Infra cit. p. 51. 



t The thickness of the "Cornstone formation" is probably not so great as 

 generally supposed. Some horizontal sections of the Geological Survey {e. g. 

 Sheet No. 3) only show a thickness of 2500 to 3500 feet, while others {e. g. 

 Sheet No. 5) show a thickness of about 6000 feet. This, however, is somewhat 

 doubtful, as the dip of the beds is unseen for upwards of a mile ; but this 

 amount may be taken as a maximum. The generally small dip of the beds, the 

 occurrence of outliers of Carboniferous, and inliers of Upper Silurian rocks, 

 occasionally over the great triangular area west of the Severn, all tend to make 

 us hesitate in. accepting the statements sometimes made of the vast thickness of 

 these beds. Quart. Journ. Greol. Soc. vol. xix. p. 476. 



\ Prof. Geikie, Quart. Journ. Geol. Soc. vol. xvi. p. S12 ; Trans. Eoy. Soc. 

 Edinb. vol. xxviii. p. 347. 



