

., 
Parr I. Sect. iv. § 2.] METAMORPHISM. 303 
them, that elevation of temperature has proceeded of which the 
metamorphism of rocks may be regarded as one result. Looking at 
the question in its broadest aspect and without reference to the special 
source of heat, we can perceive four conditions which must have 
largely determined internal rearrangements in rocks. (1) The 
temperature, from the lowest at which any change is possible up to 
that of complete fusion ; (2) the nature of the materials operated 
upon, some being much more susceptible of change from heat than 
others ; (3) the pressure under which the heat acted, the potency of 
its action being much increased with increase of pressure; (4) the 
presence of water usually containing various mineral solutions, 
whereby chemical changes might be effected which would not be 
possible in dry heat. 
Since experiment has proved that in presence of water under , 
pressure, even at comparatively low temperatures, mineral substances | 
are vigorously attacked, we may expect to find that as these con- | 
ditions abundantly exist within the earth’s crust, the rocks exposed 
- to them have been more or less altered. A large proportion of the 
accessible crust consists of sedimentary materials which were laid 
down on the ocean bottom, and which were abundantly soaked with 
sea-water even after they had been covered over with more recent 
formations. The gradual growth of the submarine accumulations 
would of course deprive the lower strata of most of their original 
water, but some proportion of it would probably remain. If, according 
to Dana, the average amount of interstitial water in stratified rocks, 
such as limestones, sandstones, and shales, be assumed to be 2°67 
per cent., which is probably less than the truth, “the amount will 
correspond to two quarts of water for every cubic foot of rock.” } 
There is certainly a considerable store of water ready for chemical 
action when the required conditions of heat and pressure are ob- 
tained. We must also remember that as the water in which the 
sedimentary formations of the crust were formed was mostly that of 
the ocean, it already possessed chlorides, sulphates, and other salts 
with which to begin its reactions. The inference may therefore be 
drawn that rocks possessing not more than 3 per cent. of interstitial 
water cannot be depressed to depths of several thousand feet beneath 
the level of the earth’s surface, and undergo great pressure and 
erushing, without suffering more or less marked internal change or 
metamorphism. | 
A metamorphosed rock is one which has suffered such a minera- 
logical rearrangement of its substance. It may or may not have been a 
erystalline rock originally. Any rock capable of alteration (and all 
rocks must be so in some degree) will, when subjected to the re- 
quired conditions, become metamorphic. The resulting structure, 
however, will, in most cases, bear witness to the original character 
of the mass. In some cases the change has consisted merely in the 
rearrangement or crystallization of one mineral originally present, as 
? Manual, 3rd ed. (1880), p, 758. 
