48 
ELEMENTS OE GEOLOGY. 
from land, and whether the water was salt,brackish, or fresh. 
Some limestones consist almost exclusively of corals, and in 
many cases it is evident that the present position of each 
fossil zoophyte has been determined by the manner in which 
it grew originally. The axis of the coral, for example, if its 
natural growth is erect, still remains at right angles to the 
plane of stratification. If the stratum be now horizontal, 
the round spherical heads of certain species continue upper¬ 
most, and their points of attachment are directed down¬ 
ward. This arrangement is sometimes repeated throughout 
a great succession of strata. 
From what we know of the 
growth of similar zoophytes 
in modern reefs, we infer 
that the rate of increase was 
extremely slow, and some of 
the fossils must have flour¬ 
ished for ages like forest- 
trees, ■ before they attained 
so large a size. During 
these ages, the water must 
have been clear and trans¬ 
parent, for such corals can 
not live in turbid water. 
Fig. *9. 
In like manner, when we 
see thousands of full-grown 
shells dispersed everywhere 
throughout a long series of 
strata, we can not doubt that 
time was required for the 
multiplication of successive 
generations; and the evi- 
Fossil covered both on the out- dence of slow accumulation 
side and inside with fossil serpiiiee. ig rendered more striking 
from the proofs, so often discovered, of fossil bodies having 
lain for a time on the floor of the ocean after death before they 
were imbedded in sediment. Nothing, for example, is more 
common than to see fossil oysters in clay, with serpulae, or 
barnacles (acorn-shells), or corals, and other creatures, at¬ 
tached to the inside of the valves, so that the mollusk was 
certainly not buried in argillaceous mud the moment it died. 
There must have been an interval during which it was still 
surrounded with clear water, when the creatures whose re¬ 
mains now adhere to it grew from an embryonic to a mature 
state. Attached shells which are merely external, like some 
of the serpulie (a) in Fig. 9, may often have grown upon an 
