551.77 305 
593.12 
563.12 
I 
The probable depths of the Gault Sea as indicated 
by its Rhizopodal Fauna 
HEN we consider the evidences of variability in the forms of 
foraminifera, and their power of adaptation to limited 
amounts of change in their environment, it may seem futile to 
attempt to attach much value to these organisms, as indices of the 
bathymetrical and other conditions of the deposit in which they are 
found. Whilst recognising this power of adaptation, however, we 
must not lose sight of the fact that marked changes are visible in 
the aspect of assemblages of foraminifera. For example, when we 
pass from material which has been laid down in clear and deep 
water in proximity to limestone cliffs, to other, and it may be, 
adjacent, and contemporary deposits, subjected to inroads of muddy 
and decomposing organic material, we shall probably find that, 
whereas in the former case the species are well-developed and thick- 
shelled, in the latter case the foraminifera will be thin-shelled and 
starved, or perhaps with tests formed, of necessity, of the minute 
sand-grains of the deposit upon which they lived. A case in point 
is afforded by the limestones and black clays of the Rhaetic of 
Wedmore Hill in Somerset (1). 
The copious records of foraminifera from known depths, 
made by the Challenger and many other important dredging and 
sounding cruises, supply us with a tangible basis for the comparison 
of types of foraminifera which are found in both recent and fossil 
accumulations. 
In consequence of the nature of the sea bottom, its temperature 
and depth being to some extent interdependent, we may gather 
many interesting facts by a due consideration of all these points. 
In the present instance those species from each zone of the 
Gault (2) have been taken, which occur also in recent deposits, where 
their known depths have been accurately recorded. These depths 
have been carefully averaged for each species selected, and the total 
mean depth for all the species in each distinct zone of the Gault has 
been taken as the probable depth of its sea bottom, In cases 
where there is a preponderance of common and well-developed 
forms, the evidence of such is considered, to the subordination of 
occasional examples, which may have been introduced into the 
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