38 DETERMINATION OF THE [Ch. IV, 



to that now prevailing. Suppose, for example, there were three 

 masses extending over every continent, the upper of chalk 

 and chloride sand ; the next below, of blue argillaceous lime- 

 stone ; and the third and lowest^ of red marl and sandstone ; 

 we must imagine that all the rivers and currents of the world 

 had been charged, at the first period, with red mud and sand ; 

 at the second, with blue calcareo-argillaceous mud ; and at a 

 subsequent epoch, with chalky sediment and chlorine sand. 



But if the ocean were universal, there could have been no 

 land to waste away by the action of the sea and rivers, and, 

 therefore, no known source whence the homogeneous sedimen- 

 tary matter could have been derived. Few, perhaps, of the 

 earlier geologists went so far as to believe implicitly in such 

 universality of formations, but they inclined to an opinion, that 

 they were continuous over areas almost indefinite; and since 

 such a disposition of mineral masses would, if true, have been 

 the least complex and most convenient for the purposes of clas- 

 sification, it is probable that a belief in its reality was often 

 promoted by the hope that it might prove true. As to the 

 objection, that such an arrangement of mineral masses could 

 never result from any combination of causes now in action, it 

 never weighed with the earlier cultivators of the science, since 

 they indulged no expectation of being ever able to account for 

 geological phenomena by reference to the known economy of 

 nature. On the contrary, they set out 5 as we have already seen, 

 with the assumption that the past and present conditions of the 

 planet were too dissimilar to admit of exact comparison. 



But if we inquire into the true composition of any stratum, 

 or set of strata^ and endeavour to pursue these continuously 

 through a country, we often find that the character of the 

 mass changes gradually, and becomes at length so different, 

 that we should never have suspected its identity, if we had not 

 been enabled to trace its passage from one form to another. 



We soon discover that rocks dissimilar in mineral compo- 

 sition have originated simultaneously; we find, moreover, 

 evidence in certain districts, of the recurrence of rocks of pro- 



