METAMORPHISM 383 
placed little by little, until it is a shell of silica, or of an ore of 
iron, or some other mineral. 
In the same way as with the shell, beds of rock may be 
slowly changed by percolating water, until, instead of a sand- 
stone or limestone, we have a bed of iron ore. As a result of 
this, not only are the par- 
ticles of the rock replaced 
(Fig. 232), but even the shells 
that are enclosed within it are 
transformed to ore. 
While other ores are some- 
times made in this way, the 
most common replacement de- 
posit is iron. This is doubt- 
less due to two facts: (1) that 
iron ores are of frequent oc- 
currence in the crust of the 
earth, and (2) that these min- 
erals are dissolved, carried, 
and deposited with ease, by : i, 
water whose temperature is es 
not high. Even now in the ee 
. : Petrified trees formerly buried in the 
crust, this action of replace- poliptoneaie focke 
ment is commonly in progress. 
CoNORETIONARY OrE Deposits. The nature of concretions 
has already been explained (p. 275). When the mineral which 
is gathered into the concretion is an ore, a valuable deposit may 
perhaps be formed. This kind of deposit is also most commonly 
of iron, for the same reasons that hold good in regard to replace- 
ment deposits of iron ore. 
A similar deposit, the segregation vein, is formed by the aggre- 
gation of ores into sheets during the metamorphism of rocks, 
where, instead of mica, hornblende, etc., iron and certain other 
ores gather into bands or beds. This is illustrated by the banding 
of metamorphic rocks. 
