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CHAP. V.] DEVONIAN PEEIOD. \X C/\l | 51 



east, and though it is doubtless continued for some distance 

 under the Shropshire Coalfield, it probably " noses out " 

 beneath the northern boundary of that county. 



What its original limits were is a much more difficult 

 question, but it undoubtedly spread some distance westward 

 over the Silurian rocks of Brecknock, Radnor, and Shrop- 

 shire ; northwards it may have stretched through parts of 

 Denbigh, Cheshire, and Derbyshire, for fragments of the 

 Tilestones which occur at the base of the Cornstone group 

 have been found in the basal Carboniferous sandstone of 

 Flint. Eastward, or rather north-eastward, it probably 

 did not extend far, for if (as we suppose) there was land 

 in that direction during Silurian times, the area of this 

 land is likely to have been much larger during the time 

 of the Lower Old Eed, which was one of upheaval. South- 

 eastward, near Berkeley in Gloucestershire, it is either 

 absent or very thin, but southward there are probably 

 large tracts of rock which were continuous with the Corn- 

 stone series, though these are concealed beneath the 

 Devonian and Carboniferous rocks. In Cornwall, however, 

 the Lower Devonians extend downward into a series of 

 grits and slates estimated to be 10,000 feet thick, and 

 these may be older than any in North Devon. 



In Scotland there are four distinct areas of Old Red 

 Sandstone: (1) in Berwickshire and the Cheviot Hills; 



(2) below and on either side of the great central coalfields ; 



(3) in the north-east, over the Orkneys, Caithness, and 

 parts of Sutherland and Ross ; (4) a small area in 

 Argyle shire. In all these there is the same general suc- 

 cession, viz., a great series of red and purple conglomerates 

 and sandstones, with dark grey flags and shales, and inter- 

 bedded sheets of felsite and porphyrite. These beds in 

 every case either pass down into Silurian rocks (as in the 

 Pentland Hills), or rest unconformably upon rocks that 

 are older than the Silurian. Above them there is always 



