18 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



derived from the shells of Foraminifera. The sediment of the Atlantic 

 Ocean is so largely constituted of one kind of foraminiferous shell, the 

 Globigerina buUoidcs, that the mud is now frequently spoken of as the 'Glo- 

 bigerina ooze'; and this material is likewise found in the Pacific and other 

 oceans and seas. 



In speaking of the composition of the ocean mud, Mr. Thomson re- 

 marks : "The dredging at 2435 fathoms at the mouth of the Bay of Biscay 

 gave a very fair idea of the condition of the bottom of the sea over an 

 enormous area, as we know from many observations which have now been 

 made, with the various sounding instruments contrived to bring up a 

 sample of the bottom. On that occasion the dredge brought up about one 

 and a half cwt. of calcareous mud. The matter contained in the dredge 

 consisted mainly of a compact 'mortar', of a bluish color, passing into a 

 thin — evidently superficial — layer, much softer and more creamy in consist- 

 ence and of a yellowish color. Under the microscope the surface layer 

 was found to consist chiefly of entire shells of Globigerina bulloides, large 

 and small, and fragments of such shells mixed with a quantity of amorphous 

 calcareous matter in fine particles, little fine sand, etc. Below the surface 

 layer the sediment becomes gradually more compact, and a slight grey 

 color, due probably to the decomposing organic matter, becomes more pro- 

 nounced, while perfect shells of Globigerina almost entirely disappear, frag- 

 ments become smaller, and calcareous mud, structureless and in a fine state 

 of division, is in greatly preponderating proportion. 



"One can have no doubt, on examining this sediment, that it is formed 

 in the main by the accumulation and disintegration of the shells of Globi- 

 gerina — the shells fresh, whole and living in the surface layer of the deposit, 

 and in the lower layers dead and gradually crumbling down by the decom- 

 position of their organic cement, and by the pressure of the layers above — 

 an animal formation in fact being formed very much in the same way as 

 in the accumulation of vegetal matter in a peat bog, by life and growth 

 above, and death, retarded decomposition and compression beneath."* 



When we are thus informed of the constitution of ocean mud, we are 

 fully prepared to learn that vast rock-formations of past ages are composed 

 of the shells of Foraminifera. D'Orbigny observes that the ' calcaire gros- 

 sier' of the Paris basin, a coarse limestone of Tertiary age, is chiefly com- 



*The Depths of the Sea, p. 410. 



