54 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



We cannot trace the true " Old Red Sandstone '* of the English 

 area further S. and E. than Frome ; and all that we can ascertain 

 with respect to its boundary-line in that direction is derived from 

 the consideration that it there consists of beds of rounded conglo- 

 merate, — or of a marginal character, — and that it does not appear in 

 Normandy, where the true coal-measure series rests immediately and 

 unconformably on the oldest slate-rocks of that country *. We 

 may hence deduce, 1st, that the boundary-line coincided with, and 

 was dependent on, the general direction of the land of the line of 

 the Channel axis, whilst the character of the conglomerate itself 

 about Frome would show that the line was not very remote. 



Here, and before the remaining portions of the Carboniferous 

 series are noticed, it may be as well to state that geological history 

 viewed largely presents throughout a frequent recurrence of two 

 distinct orders of change, dependent on cosmical laws, of the very 

 nature of which we can as yet form but vague conjectures. Of 

 these changes, the one consists in the formation at definite times of 

 elevations and depressions of the earth's crust, and hence of definite 

 geographical areas, of which we can in most instances determine, 

 even now, the main physical features : with respect to this order of 

 change, the direction in which the forces have acted would seem to 

 indicate contraction of the earth's crust. At periods intermediate 

 to these we have evidence of a process of adjustment, whereby areas 

 co-extensive with great zones, or even hemispheres, of the globe 

 have been brought into conformity with the oceanic level ; and 

 during which times marine deposits of wide range have been brought 

 unconformably over all preceding conditions of surface. In both 

 cases the progress of change was gradual. The line of demarca- 

 tion between the two distinct Lower Silurian assemblages becomes 

 one of absolute separation between the areas of the Upper Silurian 

 and Devonian faunas. The Mountain Limestone and its equiva- 

 lents, on the other hand, represents for the Palaeozoic period both 

 the greatest amount of expansion, and the greatest uniformity with 

 respect to its included fauna ; — its relations to the deposits of the 

 great Palaeozoic period are just what those of the Chalk and its 

 equivalents are to the Cretaceous. It is by this law of physical 

 change that we should be mainly guided in determining the Mainor 

 equivalents of the geological scale of depositions. 



In this way the several groups of *' Old Red Sandstone " take 

 their places as subordinate to an area of dry land, and as evidences of 

 the positions and extent of the depressions of its surface : and inas- 

 much as the geological scale is one of successive sea-beds, the forma- 

 tion must be considered as Tiors de serie, and be placed on a parallel 

 and equivalent one ; thus holding, with reference to the lower and 

 upper Palaeozoic formations, a position which corresponds with that 

 which the Purbeck-Wealden series does to the Oolitic and Cretaceous 

 groups. 



* Proc. Geol. Soe. vol. ii. p. 2. 



