THE FOSSILIFEROUS ROCKS. 23 
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 established an intimate 
and most interesting parallel- 
ism between the chalk and Fig. 8.—Organisms in the Atlantic Ooze, 
che ooze of modern oceans. chiefly Foraminifera (Globigerina and 
: : Textularia), with Polycystina and sponge- 
Both are formed essentially 1n spicules; highly magnified. (Original.) 
the same way, and the latter 
only requires consolidation 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 zdentical 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 Glodigering, and of some other organisms of little 
higher grade, that we find absolutely the same kinds or secces 
of animals in both. 
Limestone, \ike 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. 
