. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 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
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 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. 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 186
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