CRETACEOUS COCCOLITHS AND RHABDOLITHS. 49 



III. COCCOLITHS and RHABDOLITHS. 



PLATE E, FIGS. 1, 2. 



The nearest representative of the typical chalk of England and continental 

 Europe with which we are familiar, is found where the Cretaceous rocks are exposed 

 along the Upper Missouri and Niobrara rivers. It there forms bold bluffs for many 

 miles, and the name " Chalk " is very properly applied to it, from the fact that, like 

 the chalks of Europe, it is largely composed of Foraminifera and coccoliths, to 

 which, in these formations, is added the more recently discovered rhabdolith. It 

 has long been known that European chalk was largely composed of Foraminifera, 

 but it is only of late years and with improved microscopes that attention has been 

 called to these minute calcareous objects now known as "coccoliths", and yet more 

 recently that the " rhabdoliths " have been noticed. Ehrenberg first recognized 

 coccoliths associated with Foraminifera, as forming an important constituent of 

 chalk, and called them " morpholites of chalk." The name by which they are now 

 known was given them by Prof. Huxley in 1858, who found them to be characteristic 

 of many deep-sea sediments. Dr. Wallich called them coccospheres. They have been 

 carefully studied by Sorby, Hackel, Schmidt and many others, but none of them seem 

 to know their true nature or to be able to place them in any appropriate group. 



Of rhabdoliths, Dr Geo. M. Dawson, in his valuable paper, " Foraminifera, Cocco- 

 liths and Rhabdoliths from the Cretaceous of Manitoba" (Canadian Naturalist, April, 

 1874), writes as follows : " Rhabdoliths were first discovered by Dr. 0. Schmidt in 

 1872 (Ann. and Mag. Nat. Hist., 1872), in the Adriatic sea, in association with cocco- 

 liths, with which they appear to be closely allied in structure and mode of increase. 

 I do not know that they have heretofore been found in the fossil state. In samples 

 of Cretaceous limestone from Manitoba and Nebraska both coccoliths and rhabdo- 

 liths are abundant, and constitute indeed a considerable proportion of the substance 

 of the rock. The rhabdoliths agree closely with those figured by Dr. Schmidt, and 

 pass through nearly the same set of forms as those there represented. The cocco- 

 liths agree with those figured in the same place exactly, and also with those found 

 in the English chalk and recent seas. They are in a remarkably good state of 

 preservation. The average diameter of the larger among them is about .003 milli- 

 meter, which agrees very nearly with those found in other places. Dr. Gumbel 

 has discovered coccoliths in limestones of many ages, and they appear, though so 

 minute even in comparison with Foraminifera, to have played no unimportant part 

 in the fixation of calcareous matter and the building up of the crust of the earth." 



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