GEOLOGIC SYSTEMS NOT UNIVERSAL. 325 



and ceasing to subside at the same time — a comcidenco 

 which alone could produce equivalent groups of strata. 

 Subsidences in different places begin and end with utter 

 irregularity ; and hence the groups of strata thrown down 

 in them can but rarely correspond. Measured against each 

 other in time, their limits will disagree. They will refuse 

 to fit into any scheme of definite divisions. On turning to 

 the evidence, we find that it daily tends more and more to 

 justify these a priori positions. Take, as an example, the 

 Old Red Sandstone system. In the north of England this 

 is represented by a single stratum of conglomerate. In 

 Herefordshire, "Worcestershire, and Shropshire, it expands 

 into a series of strata from eight to ten thousand feet thick, 

 made up of conglomerates, red, green, and white sand- 

 stones, red, green, and spotted marls, and concretionary 

 limestones. To the south-west, as between Caermarthen 

 and Pembroke, these Old Red Sandstone strata exhibit 

 considerable lithological changes ; and there is an absence 

 of fossil fishes. On the other side of the Bristol Channel, 

 they display further changes in mineral characters and re- 

 mains. While in South Devon and Cornwall, the equiva- 

 lent strata, consisting chiefly of slates, schists, and lime- 

 stones, are so wholly different, that they were for a long 

 time classed as Silurian. When we thus see that in certain 

 directions the whole group of deposits thins out, and that 

 its mineral characters as well as its fossils change within 

 moderate distances ; does it not become clear that the 

 whole group of deposits was a local one ? And when we 

 find, in other regions, formations analogous to these Old 

 Red Sandstone or Devonian formations ; is it certain — is it 

 even probable — that they severally began and ended at the 

 same time with them ? Should it not require overwhelm- 

 ing evidence to make us believe as much ? 



Yet so strongly is geological speculation swayed by the 

 tendency to regard the phenomena as general instead of 



