52 
EI/EMENTS OF GEOLOGY. 
Fig. 15. 
Gaillonella 
ferruginea^ Ehb. 
Fig. 16. 
Fig. 17. 
Gaillonella Bacillaria parodoxa. 
distans, Ehb. a. Front view. b. Side view. 
A well-known substance, called bog-iron ore, often met 
with in peat-mosses, has often been shown by Ehrenberg to 
consist of innumerable articulated threads, of a yellow ochre 
color, composed of silica, argillaceous matter, and peroxide 
of iron. These threads are the cases of a minute microscop¬ 
ic body, called Gaillonella ferruginea (Fig. 15), associated 
with the siliceous frustules of other fresh-water alg 80 . Lay¬ 
ers of this iron ore occurring in Scotch peat bogs are often 
called “ the pan,” and are sometimes of economical value. 
It is clear that much time must have been required for the 
accumulation of strata to which countless generations of Di- 
atomacese have contributed their remains; and these discov¬ 
eries lead us naturally to suspect that other deposits, of 
which the materials have been supposed to be inorganic, 
may in reality be composed chiefly of microscopic organic 
bodies. That this is the case with the white chalk, has oft¬ 
en been imagined, and is now proved to be the fact. It has, 
moreover, been lately discovered that the chambers into 
which these Foraminifera are divided are actually often fill¬ 
ed with thousands of well-preserved organic bodies, which 
abound in every minute grain of chalk, and are especially 
apparent in the white coating of flints, often accompanied 
by innumerable needle-shaped spiculge of sponges (see Chap. 
XVII.). 
The dust we tread upon was once alive !—Byron. 
How faint an idea does this exclamation of the poet con¬ 
vey of the real wonders of nature! for here we discover 
proofs that the calcareous and siliceous dust of which hills 
are composed has not only been once alive, but almost every 
particle, albeit invisible to the naked eye, still retains the or¬ 
ganic structure which, at periods of time incalculably remote, 
was impressed upon it by the powers of life. 
Fresh-water and Marine Fossils. —Strata, whether deposited 
in salt or fresh water, have the same forms; but the imbeds 
ded fossils are very different in the two cases, because the 
aquatic animals which frequent lakes and rivers are distinct 
from those inhabiting the sea. In the northern part of the 
Isle of Wight formations of marl and limestone, more than 
