56 PALAEOZOIC TIME. [CHAP. V. 



In North Devon no base is seen ; in Cornwall there are 

 beds which may be older than any in North Devon, but 

 it is not known whether they rest on rocks of Silurian age, 

 or lie unconformably on the Ordovicians of Mevagissey. 



In comparing the rock- series of North and South Devon, 

 we may notice the prevalence of sandstones in the northern 

 area, especially in the lower half of the system, showing 

 the neighbourhood of land ; the same conclusion may be 

 drawn from a comparison of the limestones, which are 

 much thicker and more fossiliferous in South Devon, indi- 

 cating a greater depth of water in that direction. 



As regards the original extension of these Devonian 

 rocks there can be little doubt that it was chiefly in an 

 easterly direction, and that the sea in which the Middle 

 Devonians were formed shallowed toward the west and 

 south. Rocks belonging to the Middle Devonian have been 

 found in deep borings below London, and at Turnford in 

 Essex ; they occur also in Belgium and the north-east of 

 Trance, so that it is highly probable that they were con- 

 tinuous over the whole area now occupied by the English 

 Channel and the southern counties of England. In Nor- 

 mandy there is an absence of Middle and Upper Devonian, 

 though there are thick deposits of Lower Devonian age. 



3. Upper Old Red Sandstone. The precise relations of 

 the strata which have been grouped as Old Red Sandstone 

 in Brecon, Hereford, and Salop remains to be ascertained. 

 Above the Cornstone group there are signs of unconfor- 

 mity according to the Rev. W. S. Symonds, and the suc- 

 ceeding beds are brown sandstones with red marls and 

 shales. These are sometimes called Middle Old Red Sand- 

 stone, and it is possible that they represent some of the 

 marine Devonian beds ; they are probably from 3,000 to 

 4,000 feet thick, and are certainly older and distinct from 

 the true Upper Old Red Sandstone of South Wales and 

 Ireland. 



