02 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



may be said to disappear as the red-coloured strata set in. This 

 sudden change in the palaeontological and lithological aspect of the 

 rocks is now recognized as probably the result of an uprise of the 

 sea-floor and the eventual conversion of its site into a series of large 

 lakes or inland seas separated by intervening ridges of land. 



YI. OLD EED SANDSTONE. 



The vast mass of red sediments which in this country lie between 

 the top of the Upper Silurian series and the base of the Carbonife- 

 rous system can generally be clearly grouped in two great divisions. 

 Of these the lower and by far the more important, alike in thickness, 

 extent, and palseontological riches, lies conformably on the highest 

 Silurian strata. The upper division passes upward conformably 

 into the Carboniferous rocks. In Scotland and in the south-west of 

 Ireland a marked unconformability exists between the two subdivi- 

 sions. In those regions this unconformability certainly points to 

 the lapse of a considerable interval of time and to important geo- 

 graphical changes whereby the conditions and areas of deposit were 

 extensively modified. 



It is almost entirely in the lower section of the Old Eed Sandstone 

 that volcanic records have been preserved, though the upper section 

 is not entirely destitute of such chronicles. Geologists are in the 

 habit of grouping the Old Eed Sandstone and the Devonian rocks as 

 equivalent or homotaxial formations, deposited in distinct areas 

 under considerably diflPerent conditions of sedimentation. No one, 

 however, can have attempted to follow out the sequence of strata in 

 Devonshire, and to trace some analogy between the Devonian succes- 

 sion and that of the Old Eed Sandstone, without encountering many 

 difficulties for which he can find no solution. Into these problems 

 it is not needful for me here to enter. I shall briefly refer to 

 the evidence for the existence of volcanic rocks in the Devonian 

 series of the south-west of England, without attempting to decide 

 whether or to what extent the eruptions therein chronicled are to 

 be regarded as having been geologically synchronous with those of 

 the Old Eed Sandstone in the north. 



(a). Lower Old Eed Sandstone. 



In the area of the British Isles we are fortunate in the possession 

 of portions of the sites of a number of the ancient basins in which 



