551.77 305 



593.12 



563.12 



The probable depths of the Gault Sea as indicated 

 by its llhizopodal Fauna 



WHEN 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 Ehaetic 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, ami 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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