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POPULAR SCIENCE REVIEW. 
grotesque shape, or in thin films follows planes of stratification. 
It also occupies vertical cracks and joints. Besides these, cer- 
tain large cylindrical hollow fossils, growing one over another 
to the height of some yards and apparently rising contempora- 
neously with the chalk deposit, are entirely formed of flint. All 
these flints, almost without exception, show, when microscopically 
examined, marks of organic structure referable generally to 
sponges and spongiform bodies. Some idea of their appearance 
under the microscope may be obtained by referring to the 
illustration in the annexed plate (fig. g). The explanation of 
chalk, whatever it may be, must include a reasonable account of 
these contents, but as they are in some places very abundant 
and in others almost absent, it is clear that their presence is not 
absolutely necessary. That they are due to organic agency there 
cannot be a doubt, but whether they represent the protoplasm 
or Bathybius of Prof. Huxley, always at hand, and which may 
be supposed to silicify rapidly under certain conditions, or 
whether they are simple results of a quantity of silica introduced 
from time to time from without by thermal springs accumulat- 
ing round spongy bodies containing siliceous spicules (see Plate 
figs, h, i, k, 1) there is at present no sufficient evidence to 
determine. Parts of the upper chalk, as we have seen, also contain 
fragments of bryozoa, radiated animals, crustaceans, mollusks of 
all kinds, fishes, and reptiles : they even contain fragments of 
wood and a few pebbles. All these are consistent with the 
composition of the rock as a submarine mud deposited in open 
and deep water, for the recent investigations may be said to 
have cleared up all doubt as to the possibility of life of all 
known kinds in water of more than 2,000 fathoms, and the few 
foreign substances are no doubt the result of accidental drift. 
The only requirements of life, apparently indispensable both in 
water and air, are the free access of light and of certain gases 
and sufficient heat, and a temperature twelve or fifteen degrees 
above the freezing point of water appears to be sufficient for 
the rapid growth and multiplication of the Foraminifera, while 
a lower temperature is actually richer in species though infinitely 
more barren in the number of individuals. 
It is no doubt necessary before we admit that chalk is fora- 
miniferous mud that we should know its composition, and 
this has not yet been determined on a large scale. The result 
of a very simple analysis would seem to show that in some cases 
at least the foraminiferous mud or oaze contains more than 90 
per cent, of carbonate of lime. Other examples, recently sub- 
mitted by Mr. Gwynn Jeffreys, make it appear that not much 
more than .50 per cent, of some samples is of this material. It 
is clear that a few isolated analyses are not sufficient in a 
question of this magnitude, and thus a certain amount of doubt 
