Vol. 67.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxiii 



(6) Examples. 



The simple and stately progression of events thus outlined, from 

 depression to uplift and back again to depression, can rarely 

 be exemplified in completeness from the succession of British 

 Formations. There are necessarily many checks, variations in 

 the rate of movement, and alterations in both rate and type of 

 deposition. Thus, a movement of subsidence or of elevation has 

 rarely been uninterrupted by pauses, or even by reversal of 

 movement. The great Caledonian movement had at least three 

 periods of high intensity, with pauses and reversals. The 

 movement was at times so slow that it was overtaken by 

 deposition, while at other times deposits were swamped by its 

 rapidity. Besides this, as has been frequently pointed out, certain 

 elasses of the deposits are especially liable to destruction during 

 submergence or emergence. When to this is added the possible 

 influence of isostasy, allowing the rapidly denuded land to rise and 

 the heavily loaded sea-bed to sink, the results of any particular 

 geographical phase become hard to follow and to link into a 

 connected scheme. 



We may perhaps trace in the Coal Measures the influence of the 

 last-named cause. Although the story of the Carboniferous Period is 

 one of the shallowing of an area by upward movement, there are facts 

 which are best explained by occasional reversal. If we accept 

 the ' growth in situ ' theory of coal-seams, this contention needs no 

 further support, for the entire history of these strata would be 

 explicable by slow irregular subsidence, accompanied by deposition 

 sufficiently rapid to fill the hollows as fast or nearly as fast as 

 they were formed ; while the coal-seams would be explicable as the 

 results of prolonged pauses in the general downward motion. Even 

 if we do not accept the theory of growth in situ, the overlap in 

 Britain of the Lower by the Upper Coal Measures is evidence of an 

 area of deposit temporarily expanding. That the movement was 

 not quite a simple one, however, but was accompanied by con- 

 siderable folding of the older Coal Measures, has been abundantly 

 proved by Mr. W. F. Clark in his new interpretation of the 'Symon 

 Fault' of the Coalbrookdale Coalfield. In a paper published in 

 our Journal he has shown that this phenomenon is not due to an 

 eroded valley as supposed by Scott, but to the planation of a series 

 of asymmetrical folds, on the denuded edges of which the newer 

 Coal Measures were subsequently laid down. 



In our applications of Physiography to Geology we are not 

 vol. lxvii. / 



