1858.] 071 the Rhizopod Type of Animal Life. 503 



siderations which connect themselves with the study of this group 

 having thus been noticed, its geological importance has in the last 

 place to be alluded to. Traces, more or less abundant, of the ex- 

 istence of Foraminifera are to be found in calcareous rocks of nearly 

 all geological periods ; but it is towards the end of the secondarj', 

 and at the beginning of the tertiary period, that the development 

 of this group seems to have attained its maximum. Although there 

 can be no reasonable doubt that the formation of Chalk is partly 

 due to the disintegration of corals and larger shells, yet it cannot 

 be questioned that in many localities a very large proportion of 

 its mass has been formed by the slow accumulation of foraminiferous 

 shells, sometimes preserved entire, sometimes fragmentary, and 

 sometimes almost entirely disintegrated. The most extraordinary 

 manifestation of tliis type of life, however, presents itself in the 

 "nummulitic limestone," which may be traced from the region 

 of the Pyrenees, through that of the Alps and Apennines, into Asia 

 Minor, and again through Northern Africa and Egypt, into Arabia, 

 Persia, and Northern India, and thence (it is believed) through 

 Thibet and China, to the Pacific, covering very extensive areas, 

 and attaining a thickness in some places of many thousand feet ; 

 another extensive tract of this nummulitic limestone is found in the 

 United States. A similar formation, of less extent but of great 

 importance, occurs in the Paris basin ; and it is not a little remark- 

 able that the fine-grained and easily -worked limestone, which affords 

 such an excellent material for the decorated buildings of the French 

 metropolis, is entirely formed of an accumulation of minute fora- 

 miniferous shells. Even in the nummulitic limestone, the matrix 

 in which the nummulites are imbedded, is itself composed of minute 

 Foraminifera, and of the comminuted fragments of larger ones. The 

 remarkable discovery has been recently made by Prof. Ehrenberg, 

 that the green and ferruginous sands which present themselves in 

 various stratified deposits, from the Silurian to the Tertiary epoch, 

 but which are especially abundant in the Cretaceous period, are 

 chiefly composed of casts of the interior of minute shells of Fora- 

 minifera and Mollusca, the shells themselves having entirely disap- 

 peared. The material of these casts, which is chiefly silex, coloured 

 by silicate of iron, has not merely filled the chambers and their 

 communicating passages, but has also penetrated, even to its mi- 

 nutest ramifications, that system of interseptal canals, whose exist- 

 ence, first discovered by Dr. C in Nummulites, has been detected 

 also in many recent Foraminifera allied to these in general plan of 

 structure. And it is a very interesting pendant to this discovery, 

 that a like process has been shown by Prof. Bailey, to be at present 

 going on over various parts of the sea bottom of the Gulf of Mexico 

 and the G ulf Stream ; casts of Foraminifera in green sand being 

 brought up in soundings with living specimens of the same types. 



[W. B. C] 



