OLD RED SANDSTONE AND DEVONIAN. II3 



OLD RED SANDSTONE AND DEVONIAN. 



The precise correlation of the strata which intervene 

 between the Silurian and the Carboniferous rocks has for 

 a long time been one of the vexed questions of Geology. 

 On most geological maps three large areas of country are 

 similarly coloured to represent the rocks formed during this 

 interval. In one area, including parts of the counties of 

 Monmouth, Hereford, Brecknock, etc., the rocks are termed 

 Old Red Sandstone ; in two other areas, including part of 

 North Devon and the greater part of Cornwall and South 

 Devon, the rocks are termed Devonian. 



The relations between the Old Red Sandstone and the under- 

 lying Silurian and overlying Carboniferous rocks have been 

 long established. In both instances a perfect conformity 

 exists. But only in' recent years has it been fully realized 

 that there is a great unconformity between the Upper and 

 the Lower Old Red Sandstone. 



On the other hand, the Devonian rocks, which were rescued 

 from the chaos of " Greywacke " by Sedgwick and Murchison, 

 were shown by Lonsdale to contain fossils of a character 

 intermediate between those of the Silurian and Carboniferous 

 rocks ; and hence the Devonian rocks were supposed to be 

 equivalent to the Old Red Sandstone. 



The term Old Red Sandstone has, in Britain, been restricted 

 to those deposits of red sandstone, etc., in which few, if any, 

 marine fossils occur, and which, both from their lithological 

 characters and organic remains, appear to have been laid 

 down in inland areas of deposit. The Devonian rocks com- 

 prise slates and limestones, as well as red sandstones, and in 

 the former rocks the fossils are of a decidedly marine type. 

 Hence, in considering the physical conditions under which 

 the strata were deposited, it has always been a puzzle to 

 understand how these different types of strata were con- 

 temporaneously deposited, because they are not separated 

 geographically by a very wide interval. If we regard the 

 Old Red Sandstone as entirely formed in fresh waters, we 

 require a considerable barrier to separate the purely marine 

 area of the Devonian slates and limestones from the lacustrine 

 sands of the Old Red period ; and such a barrier (if it ever 

 existed) must be drawn somewhere between the Mendip and 

 Quantock Hills, and there is no physical evidence to support it. 



The stratigraphical relations of the Old Red Sandstone 

 with the rocks above and below, have been pointed out ; those 



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