THE FOSSILIFEROUS ROCKS. 



23 



Fig. 8. — Organisms In the Atlantic Ooze, 

 chiefly Foratninifera {Qlobigerina and 

 TexUdarid), with Polycyistina and sponge- 

 spicules; highly magnified. (Original.) 



and a certain proportion of the flinty cases of minute animal 

 and vegetable organisms (^Polycystina and Diatoms). Though 

 many of the minute animals, 

 the hard parts of which form 

 the ooze, undoubtedly live at 

 or near the surface of the sea, 

 others, probably, really live 

 near the bottom ; and the ooze 

 itself forms a congenial home 

 for numerous sponges, sea- 

 lilies, and other marine ani- 

 mals which flourish at great 

 depths in the sea. There is 

 thus estabhshed an intimate 

 and most interesting parallel- 

 ism between the chalk and 

 the ooze of modern oceans. 

 Both are formed essentially in 

 the same way, and the latter 

 only requires consohdation to become actually converted into 

 chalk. Both are fundamentally organic deposits, apparently 

 requiring a great depth of water for their accumulation, and 

 mainly composed of the remains of Foraminifera, together 

 with the entire or broken skeletons of other marine animals of 

 greater dimensions. It is to be remembered, however, that the 

 ooze, though strictly representative of the chalk, cannot be 

 said in any proper sense to be actually identical with the for- 

 mation so called by geologists. A great lapse of time separates 

 the two, and though composed of the remains of representative 

 classes or groups of animals, it is only in the case of the lowly- 

 organised Giobige7'ince, and of some other organisms of little 

 higher grade, that we find absolutely the same kinds or species 

 of animals in both. 



Limestone, like chalk, is composed of carbonate of lime, 

 sometimes almost pure, but more commonly with a greater or 

 less intermixture of some foreign material, such as alumina or 

 silica. The varieties of limestone are almost innumerable, 

 but the great majority can be clearly proved to agree with 

 chalk in being essentially of organic origin, and in being more 

 or less largely composed of the remains of living beings. In 

 many instances the organic remains which compose limestone 

 are so large as to be readily visible to the naked eye, and the 

 rock is at once seen to be nothing more than an agglomera- 

 tion of the skeletons, generally fragmentary, of certain marine 

 animals, cemented together by a matrix of carbonate of lime. 



