320 



ANIMAL ORIGIN OF WHITE CHALK. 



[Ch. XVII. 



marine area, of the calcareous matrix of the chalk, as well as of the 

 imbedded fossils, would take place more readily in consequence of 

 the low specific gravity of the shells of mollusca and zoophytes, when 

 compared with ordinary sand and mineral matter. The mud also de- 

 rived from their decomposition would be much lighter than argillace- 

 ous and inorganic mud, and very easily transported by currents, espe- 

 cially in salt water. 



But the analogy of existing coral reefs would better illustrate such 

 formations as the Oolitic limestones, to be described in Chapters XX. 

 and XXL, which consists in great part of compact rock, than the soft 

 and unconsolidated white chalk. A new light has recently been 

 thrown upon the origin of the latter deposit by the deep soundings 

 made in the North Atlantic, previous to laying down, in 1858, the 

 electric telegraph between Ireland and Newfoundland. At depths 

 sometimes exceeding two miles, the mud forming the floor of the 

 ocean was found, when examined by Professor Huxley, to be almost 

 entirely composed (more than nineteen-twentieths of the whole) of 

 minute Rhizopods, or foraminiferous shells of the genus Globigerina, 

 especially the species of Globigerina bulloides (see fig. 281). In the 

 remainder of the mud the organic bodies next in quantity were the 

 siliceous shells called Polycystinece, and next to them the siliceous 

 skeletons of plants called- Diatomacece (figs. 282, 283, 284), and 

 occasionally some siliceous spiculse of sponges (fig. 285), were inter- 

 mixed. 



Fig. 281. 



Fig. 282. 



Fig. 283. Fig. 284. Fig. 285. 



Organic bodies forming the ooze of the bed of the Atlantic at great depths. 

 Fig. 281. Globigerina bulloides. Calcareous EMzopod. 



282. Actinocyclas. ) 



283. Pinnularia. V Siliceous Diatoms. 



284. Eunotia bidens. ) 



285. Spicula of sponge, Siliceous sponge. 



In 1860, shells of the same Globigerina were observed by Sir Leo- 

 pold MacClintoch and Dr. Wallich, during the cruise of the " Bulldog," 

 to form, over other wide areas of the Atlantic, a proportion of about 

 95 per cent, of the mud, both between the Faroe Islands and Iceland, 

 and between Iceland and Greenland. The consistency of the ooze 

 brought up from great depths in these areas is described as akin to 

 that of putty. On the surface were found living Globigerina, while 

 immediately below were countless calcareous grains, the relics of by- 

 gone generations. Each of these grains, as will be seen by the mag- 

 nified drawing, instead of being solid, consists of a collection of cells, 

 and as similar Globigerinse form a large part of the white chalk, their 



