290 
ELEMENTS OF GEOLOGY. 
mountain chains of the principal continents, he will at once 
perceive that the present Atlantic, Pacific, and Indian Oceans 
are geographical terms, which must be wholly without mean¬ 
ing when applied to the Eocene, and still more to the Creta¬ 
ceous Period; so that to talk of the chalk having been un¬ 
interruptedly forming in the Atlantic from the Cretaceous 
Period to our own, is as inadmissible in a geographical as in 
a geological sense. 
Chalk-flints.—The origin of the layers of flint, whether 
in the form of nodules, or continuous sheets, or in veins or 
cracks not parallel to the stratification, has always been 
more difficult to explain than that of the white chalk. But 
here, again, the late deep-sea soundings have suggested a 
possible source of such mineral matter. During the cruise 
of the “ Bulldog,” already alluded to, it was ascertained that 
while the calcareous Globigermce had almost exclusive pos¬ 
session of certain tracts of the sea-bottom, they were wholly 
wanting in others, as between Greenland and Labrador. Ac¬ 
cording to Dr. Wallich, they may flourish in those spaces 
where they derive nutriment from organic and other matter, 
brought from the south by the warm waters of the Gulf 
Stream, and they may be absent where the effects of that 
great current are not felt, l^ow, in several of the spaces 
where the calcareous Rhizopods are wanting, certain micro¬ 
scopic plants, called Diatomaceoe^ above mentioned (Figs. 
233-235), the solid parts of which are siliceous, monopolize 
the ground at a depth of nearly 400 fathoms, or 2400 feet. 
The large quantities of silex in solution required for the 
formation of these plants may probably arise from the disin¬ 
tegration of feldspathic rocks, which are universally distrib¬ 
uted. As more than half of their bulk is formed of siliceous 
earth, they may afford an endless supply of silica to all the 
great rivers which flow into the ocean. We may imagine 
that, after a lapse of many years or centuries, changes took 
place in the direction of the marine currents, favoring at 
one time a supply in the same area of siliceous, and at an¬ 
other of calcareous matter in excess, giving rise in the one 
case to a preponderance of Globigerinse, and in the other of 
Diatomaceae. These last, and certain sponges, may by their 
decomposition have furnished the silex, which, separating 
from the chalky mud, collected round organic bodies, or 
formed nodules, or filled shrinkage cracks. 
Pot-stones.—A more difficult enigma is presented by the 
occurrence of certain huge flints, or pot-stones, as they are 
called in Norfolk, occurring singly, or arranged in nearly 
continuous columns at right angles to the ordinary and hor- 
