CHAPTER VII 



GEOLOGIC DISTRIBUTION 



It is not possible in a short chapter to take up the detailed 

 geologic distribution of any but the larger groups of the 

 foraminifera. The nude types must be very old and of course 

 have left no traces in the rocks. Likewise those forms which 

 have developed chitin alone are almost without hope of being 

 preserved in recognizable form unless they might somewhere 

 have developed in such numbers as to have made a recognizable 

 mass. At least they are not known from the earlier rocks. The 

 arenaceous group, especially those which have very definite tests, 

 are capable of good preservation if conditions are favorable. 

 The chitinous base and more or less elastic cementing materials 

 make them much less capable of holding their form under pres- 

 sure than is the case with the calcareous forms. In Cretaceous 

 and even Tertiary sediments which although unconsolidated 

 have been subjected to stresses and shearing the calcareous 

 foraminifera will be found to have held their shape well while 

 the arenaceous forms with them will often be greatly twisted 

 out of shape. The more firmly cemented forms such as the 

 Textulariidae have been found in the early formations and a 

 careful study of such sediments for arenaceous forms will un- 

 doubtedly bring many more to our knowledge. The calcareous 

 forms are well preserved as a rule unless there has been leaching 

 or the material turned to greensand before it was fully fossilized. 



In the very early rocks microscopic objects have been found 

 which have been assigned to the foraminifera, usually to the 

 genus Orbulina which really is a specialized end form and in no 

 sense a primitive one and whose geologic history is not long. 

 In the pre-Cambrian quartzites of France, Cayeux has recorded 

 microscopic objects which he has referred to the foraminifera 

 although noting that the structure is obliterated and that some 

 of them may be equally well referred to the radiolaria. The 



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