MIDDLE CHALK—CONDITIONS OF DEPOSIT. 
551 
this I mean that the water is not likely to have been anywhere 
over 1,000 fathoms in depth, and that the central part of the sea 
was not more than 200 or 250 miles from the nearest land. 
At the same time, the purity of the chalk of this stage is very 
great, and the seams of marl which do occur are very thin ; 
whence we must infer that very little terrigenous material was, 
as a rule, carried into the Turonian Sea. This purity of the deposit 
may have been due to several causes. We know that the region 
was undergoing subsidence, a process which checks detrition 
and leads to the drowning of river valleys, and their conversion 
into fiords or lochs, with the consequence that less sediment is 
carried into the open sea. Again, the nearest tracts of land appear 
to have been of the nature of islands or of promontories, and, 
consequently, were not likely to produce any large rivers. Lastly, 
though there seems to have been a large area of land to the west 
of Great Britain, it is quite possible that the main drainage of this 
land was directed westward or southward, so that no mud-bearing 
rivers emptied themselves through its eastern shores. 
It is fairly certain also that the area in which the Middle Chalk 
of England was accumulated was part of the deeper and more 
central portion of the Anglo-Parisian sea ; but the attempt to 
form some idea of the actual depth of this part of the sea is beset 
with difficulties. It is fairly safe to say that it was deeper than 
it had ever been during the epoch of the Lower Chalk, but a sub¬ 
sidence of 100 feet may have been sufficient to submerge much 
land and ensure the formation of a purer kind of deposit, and 
100 feet is less than 17 fathoms. 
In attacking this problem we can, of course, only argue by 
analogy, and cannot hope to find any certain criterion. As in 
other cases, there are two sets of facts, which provide us with a 
certain amount of evidence—(1) microscopical inquiry into the 
relative quantities and dimensions of the detrital minerals found 
in the chalk and the ooze ; (2) the included organisms and the 
analogies which they suggest. 
Mr. Hill has dealt with the mineral particles which exist in 
this part of the Chalk in Chapter xliii., to which the reader is 
referred, but attention may here be directed to certain points. 
First it is noteworthy that the total amount of extraneous mineral 
matter in these chalks is exceedingly small (see tables on pp. 513 
and 519 ), and, of course, the quantity of recognisable mineral 
particles is still smaller. Thus, in no specimen of the Rhynchonella 
Cuvieri zone (including the Melbourn Bock) does the entire “ coarse 
