486 ON THE SUCCESSIVE FORMS 



fered for the solution of many obvious difficulties re- 

 specting the positions of the lowest secondary strata. 



I must yet remark on these depressions and ele- 

 vations, or changes of relative level in the sea and 

 land, that accustomed, partly through the violence of 

 volcanic action, and partly through the influence of 

 the term revolution, to consider such changes as vio- 

 lent and brief, we are probably depriving ourselves of 

 the means of explaining many appearances among 

 these, and perhaps other strata. It is quite con- 

 ceivable, that such a change should have been ex- 

 tremely gradual and tedious, resembling the slow 

 changes of level known on the coast of Italy, and ap- 

 parently in our own island. In this case the great 

 depth of the coal strata is easily explained, as I have 

 already hinted ; and thus might we also account for 

 the alternations of marine strata so often suspected in 

 these deposits. Thus also we might explain certain 

 phenomena, real or supposed, belonging to the lias 

 and its animals ; because if these were deposited on 

 its maritime boundary at certain points, a gradual sub- 

 sidence might have caused them to occupy a large 

 horizontal space. 



I have thus brought down this enquiry to a state 

 of things, in which a portion, both of the primary 

 and of the older secondary strata formed the dry land 

 of the earth, as they had done before the last revo- 

 lution ; but, in which, a large portion of these also, 

 which had once been elevated, were again depressed 

 beneath the waters, together with the productions 

 which occupied their surface. This is the fourth great 

 revolution which the earth appears to have expe- 

 rienced ; the three former consisting, as far as the ap- 

 pearances show, in elevations of the bottom of the 

 ocean, and this one in a depression of the dry land ; 

 but under the reservations just made. And I may 



