168  GEOGNOSY. [Boor Il. 
gradually obliterated by the passage through them of percolating 
water dissolving and redepositing calcium carbonate. We can thus 
understand how a mass of crystalline limestone may have been 
produced from one formed out of organic remains without the action 
of any subterranean heat, but merely by the permeation of water 
from the surface.* | 
Chalk—a white soft rock, meagre to the touch, soiling the 
fingers, formed of a fine calcareous flour derived from the remains of 
Foraminifera, echinoderms, molluscs, and other marine organisms. 
By making thin slices of the rock and examining them under the 
microscope, Sorby has found that Foraminifera, particularly Globe- 
gerina, and single detached cells of comparatively shallow-water 
torms, probably constitute less than half of the rock by bulk (ig. 
14), the remainder consisting of detached prisms of the outer 
ealcareous layer of Inoceramus, fragments of Ostrea, Pecten, echino- 
derms, spicules of sponges, &c. It is not quite like any known 
modern deep-sea deposit. 
Crinoidal (Enerinite) Limestone—a rock composed in 
great part of crystalline joints of encrinites, with Moramensfera, 
corals, and molluscs. It varies in colour from white or pale grey, 
through shades of bluish-grey (sometimes yellow or brown, less 
commonly red) to a dark-grey or even black colour. It is abundant 
among Paleozoic formations, being in Western Europe especially 
characteristic of the lower part of the Carboniferous system. 
(2.) Siliceotis. 
Silica is directly eliminated from both fresh and salt water by the 
vital growth of plants and animals. (Book III. Part I. Section iii.) 
Diatom-earth (Infusorial earth)—a siliceous deposit formed 
chiefly of the frustules of diatoms, Jaid down both in salt and in fresh 
water. Wide areas of it are now being deposited on the bed of the 
South Pacific (Diatom-ooze, Fig. 173). In Virginia, United States, an 
extensive tract occurs covered with diatom-earth toa depth of 40 feet. 
Ht is used as ¢r¢polt powder for polishing purposes. 
Radiolarian-ooze—an abysmal marine deposit consisting mainly 
of the remains of siliceous radiolarians and diatoms (Fig. 181). It is 
further referred to in Book III. Part IT. Section iii. | 
Flint (Chert) has been already (p. 117) described, but should 
find a place also here from its evident connection with organic 
agency. It frequently encloses sponges, echini, shells, &e., and has 
evidently formed round these on the sea-floor, and has replaced their 
original calcium-carbonate. In some cases, as in the spicules of 
sponges, it has had a directly organic origin, having been secreted 
from sea-water by the living organisms; in other cases, where for 
example we find a calcareous shell, or echinus, or coral, converted 
into silica, it would seem that the substitution of silica for calcium- 
! See Dana’s Coral and Coral Islands, p, 354, 

