CHAPTER V. 



OLD RED SANDSTONE AND DEVONIAN. 



THERE is much uncertainty about the classification 

 and correlation of the rocks which intervene between 

 the Silurian and the Carboniferous systems, and this un- 

 certainty mainly arises from the fact that there is no 

 locality in Britain where an unbroken series of these inter- 

 vening rocks can be studied. In several areas the Silurian 

 is seen to pass upward into a series of red and purple 

 sandstones (Old Red Sandstone), but in every case there is 

 a break before the Carboniferous series is reached. Again, 

 there is one area (North Devon) where Carboniferous rocks 

 rest conformably on a series of strata containing a peculiar 

 fauna which stamps them as of intermediate age between 

 Silurian and Carboniferous, but the base of these (Devonian) 

 rocks is not seen, so that we do "not know what they rest 

 upon, and it is possible that a considerable thickness of rocks 

 intervenes between the Lower Devonian Sandstones and 

 the Silurians of Upper Ludlow type, if such beds exist 

 beneath North Devon. 



The Old Red Sandstone is divided into a Lower and an 

 Upper Series, with a gap between them. The greater part 

 of the Devonian system was doubtless deposited during 

 the period which this gap represents, though its lowest 

 portion may have been coeval with part of the Lower Old 

 Red, and its uppermost portion is generally acknowledged 

 as the equivalent of the Upper Old Red. 



