ORIGIN OF THE OOLITE AND LIAS. 
365 
Nevertheless, some of the clays and intervening limestones 
do retain, in reality, a pretty uniform character for distances 
of from 400 to 600 miles from east to west and north to south. 
In order to-account for such a succession of events, we may 
imagine, first, the bed of the ocean to be the receptacle for 
ages of fine argillaceous sediment, brought by oceanic cur- . 
rents, w^hich may have communicated with rivers, or with 
part of the sea near a wasting coast. This mud ceases, at 
length, to be conveyed to the same region, either because the 
land which had previously suffered denudation is depressed 
and submerged, or because the current is deflected in another 
direction by the altered shape of the bed of the ocean and 
neighboring dry land. By such changes the water becomes 
once more clear and fit for the growth of stony zoophytes. 
Calcareous sand is then formed from comminuted shell and 
coral, or, in some cases, arenaceous matter replaces the clay; 
because it commonly happens that the finer sediment, being 
first drifted farthest from coasts, is subsequently overspread 
by coarse sand, after the sea has grown shallower, or when 
the land, increasing in extent, whether by upheaval or by¬ 
sediment filling up parts of the sea, has approached nearer 
to the spots first occupied by fine mud. 
The increased thickness of the limestones in those regions, 
as in the Alps and Jura, where the clays are comparatively 
thin, arises from the calcareous matter having been derived 
from species of corals and other organic beings which live in 
clear water, far from land, to the growth of which the influx 
of mud would be unfavorable. Portions therefore of these 
clays and limestones have probably been formed contempo¬ 
raneously to a greater extent than we can generally prove, 
for the distinctness of the species of organic beings would 
be caused by the difierence of conditions between the more 
littoral and the more pelagic areas and the different depths 
and nature of the sea-bottom. Independently of those as¬ 
cending and descending movements which have given rise 
to the superposition of the limestones and clays, and by 
which the position of land and sea are made in the course of 
ages to vary, the geologist has the difficult task of allowing 
for the contemporaneous thinning out in one direction and 
thickening in another, of the successive organic and inorgan¬ 
ic deposits of the same era. 
