LOWER CHALK—DEPTH OF THE SEA. 
355 
Depth of Water. 
Coming now to the difficult problem of endeavouring to gauge 
the depth of water beneath which the Lower Chalk was accumu¬ 
lated, we may first indicate some general considerations, and then 
discuss such actual evidence as is available. 
First, then, the mere change from Gault mud or from malm- 
stone to glauconitic sand and marly chalk does not necessarily 
mean an increase of depth, but only a change in the direction of 
mud-bearing currents. Opposite the mouths of great rivers 
terrigenous mud may be deposited at depths of over 1,000 fathoms, 
as, for instance, in the Arabian Sea and in the Bay of Bengal, 
where, but for the Indus and the Ganges, Globigerina ooze would 
doubtless be forming. 
It is true that in the case of the Lower Chalk there is every reason 
to suppose that the change was brought about by continued subsi¬ 
dence, which widened the area of the sea at the expense of the 
surrounding land, lessened the volume of the rivers, and eventually 
opened up passages into other seas, by means of which the direction 
of the currents was greatly altered. Still, the actual amount of 
vertical subsidence which brought about the final change need 
not have been great; 200 feet is only a little over 33 fathoms, and 
200 feet might well have been sufficient to submerge much land 
and to breach important isthmuses. Yet, if the depth of the water 
during the formation of the highest part of the Folkestone Gault 
was 300 fathoms, w r e should not regard an increase of 33 fathoms 
as a great accession of depth. 
The size of the grains of mineral detritus in a deposit is rather 
an indication of distance from land than of the depth of water, 
except so far as the one depends on the other. At the base of the 
Chalk Marl large grains of quartz often occur from 2 to 3 mm. in 
diameter, and these are not likely to occur so frequently at a distance 
of more than 100 miles from land or at depths of more than 500 
fathoms. They indicate the action of strong currents, but it is 
noticeable that in the south-east of England the mineral grains 
rapidly decrease in size upward ; that those found in the grey 
marls seldom exceed * 5 mm.; while in the higher beds they seldom 
exceed '05—a size which might occur in almost any depth. 
We may next consider whether any of the fossil remains will 
afford us more definite evidence. The more highly organised 
creatures, such as Fish and Cephalopods, are of little value in this 
inquiry, because of their capacity to move about freely, and the 
same may be said of the Crustacea, for such recent Brachyuran 
forms as have kindred genera in the Lower Chalk range down to 
500 or 600 fathoms, while the Macruran genera range to over 1,000 
fathoms, and Cirripedes to much greater depths. 
Gasteropoda .—More assistance might be expected from Gastero- 
pods, most of which spend their adult lives on the sea floor, but many 
genera are now known to have such a great range in depth that 
they are of little use as bathymeters. Thus, for instance, the 
4219. A a 
