50 
POPULAE SCIENCE EEVIEW. 
jhas the mud of the sea-bottom become formed than it begins 
to be covered. When covered it begins to consohdate and 
parts with some of its excess of moisture. In this state it may 
long remain^ but ultimately it gets covered up with coat after 
coat of similar or different material, and by some of those de- 
pressions that constantly affect large portions of the earth^s 
crust it sinks down and acquires an equable temperature 
belonging to its depth. Thus placed it is subjected to the 
influence of such polar forces as act within the eartVs surface. 
It is also subject to enorinous pressm*e greatly increased in the 
event of an upheaval. During* all this time water acts. It 
helps the half-formed mass to become a definite solid ; it 
penetrates every pore, and ciystallizes the yet shapeless atoms 
of the ancient mud; it fills up all the crevices; it takes 
away here and replaces there ; it separates out small propor- 
tions of foreign bodies, collecting them into one place; it 
converts the shapeless mass into strata ; it forms bands that 
are among the strata, but independent of them ; it even helps 
the separation of metals, and places them in a certain order 
in the vacant spaces. 
Far from being obstructed by the compactness produced by 
superincumbent weight, water acts with more vigour as the 
pressure is greater. Where the depth is very great and the 
temperature high, as at a depth equivalent to the height of 
Mont Blanc and a temperature of 680° Fahr., the conditions 
would be favourable for the formation of granite. At smaller 
depths and lower temperature all intervening modifications 
and metamorphoses may be produced. 
Such is metamorphism in geology — a change in rocks in- 
duced by the action of such temperature, such pressime, and 
such a percolation of water as we believe to exist within a 
moderate distance from the earth^s surface ; a change not 
involving destruction, but on the other hand essentially for- 
mative. It is a change neither violent nor convulsive, but 
normal and regular ; imposed originally on matter, adapted to 
the conditions under which the earth was originally formed, 
carried out continually and incessantly, formerly as now, and 
now as formerly. This is the idea of metamorphism that com- 
mends itself to those who would regard geology as a true his- 
tory of the earth, and who would if possible explain phenomena 
by such a causation as is consistent with their knowledge and 
experience. 
The foregoing are a few simple facts tending to illustrate 
the position that metamorphosis is a great fact in nature; 
that it is the rule, and not the exception ; that it is con- 
stantly going on ; and that its natural result is the production 
of such a world as that in which we live. 
