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 chloritic 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 chloritic 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, 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 pf rocks of pre- 
