CONSOLIDATION OF STRATA. 
61 
must be produced by vital agency as in the case of coral 
reefs. 
In such reefs, large masses of limestone are formed by the 
stony skeletons of zoophytes; and these, together with shells, 
become cemented together by carbonate of lime, part of 
which is probably furnished to the sea-water by the decom¬ 
position of dead corals. Even shells, of which the animals 
are still living on these reefs, are very commonly found to be 
incrusted over with a hard coating of limestone. 
If sand and pebbles are carried by a river into the sea, and 
these are bound together immediately by carbonate of lime, 
the deposit may be described as of a mixed origin, partly 
chemical, and partly mechanical. 
Now, the remarks already made in Chapter II. on the orig¬ 
inal horizontality of strata are strictly applicable to mechan¬ 
ical deposits, and only partially to those of a mixed nature. 
Such as are purely chemical may be formed on a very steep 
slope, or may even incrust the vertical walls of a fissure, and 
be of equal thickness throughout; but such deposits are of 
small extent, and for the most part confined to vein-stones. 
Consolidation of Strata. —It is chiefly in the case of calca¬ 
reous rocks that solidification takes place at the time of dep¬ 
osition. But there are many deposits in which a cementing 
process comes into operation long afterwards. We may 
sometimes observe, where the water of ferruginous or calca¬ 
reous springs has flowed through a bed of sand or gravel, 
that iron or carbonate of lime has been deposited in the in¬ 
terstices between the grains or pebbles, so that in certain 
places the whole has been bound together into a stone, the 
same set of strata remaining in other parts loose and inco¬ 
herent. 
Proofs of a similar cementing action are seen in a rock at 
Kelloway, in Wiltshire. A peculiar band of sandy strata be¬ 
longing to the group called Oolite by geologists may be 
traced through several counties, the sand being for the most 
13art loose and unconsolidated, but becoming stony near Kel¬ 
loway. In this district there are numerous fossil shells which 
have decomposed, having for the most part left only their 
casts. The calcareous matter hence derived has evidently 
served, at some former period, as a cement to the siliceous 
grains of sand, and thus a solid sandstone has been produced. 
If we take fragments of many other argillaceous grits, re¬ 
taining the casts of shells, and plunge them into dilute mu¬ 
riatic or other acid, we see them immediately changed into 
common sand and mud; the cement of lime, derived from the 
shells, having been dissolved by the acid. 
