METAMOEPHISM. 
45 
that into marble^ is a phenomenon sufficiently well known to 
be quoted as an example of gradual change. The passage of 
sand into sandstone_, and of that into a rock so compact_, 
hard^ and crystalline^ as to become qnarzite^ is equally familiar 
to geologists. Clays_, in like manner_, pass into shales very 
gradually. Some shales put on the character of slate^ and 
slates are but modified shales. All these are examples of 
repeated metamorphism^ first producing and then modifying 
the simplest_, most familiar^ and most abundant rocks. 
In truths no one can examine a stone from a quarry^ a clod 
from the field_, or a specimen from a mine^ without being made 
aware that he sees a changed substance^ — changed^ perhaps_, 
from something common and simple enough^ — changed^ it may 
be_, into something equally simple and equally common^ — but 
always a new thing, not the rock deposited, but the rock 
derived. And this is one of the great lessons to be learned 
from geological pursuits. The earth is not a planet once 
formed after a certain fashion, and ever since remaining in 
the same state. The minerals beneath our feet are just as 
much in perpetual growth and development as the animals and 
the plants on the surface. They live their life and suffer their 
death. Eocks are developed, not merely accumulated; and 
this is the case with every variety of limestone, sandstone, and 
clay, as well as the slates and schists usually described as 
metamorphic. 
Metamorphism, then, is not, in this sense, a marvellous and 
mysterious principle, interfering with the natural order of 
things, and producing unknov/n rocks in an unknown manner. 
It is a term which may and must be applied, if it be applied at 
all, to every rock of the earth A crust, aqueous as well as 
igneous. It is not too much to say that no such thing 
exists in nature as an unchanged accumulation of mineral sub- 
stances. The nearest approach would be a heap of quartz 
sand ; but even where nothing but quartz sand is present there 
are generally portions harder than the rest, portions where 
water has bubbled through and carried with it foreign material 
of various kinds, acting* as a cement, and leaving behind 
ample proof of its passage. 
But no doubt there are degrees of metamorphism, and we 
have as yet considered only the most simple. The passage of 
rocks, once clay, into the state of roofing slate, and the pas- 
sage of other rocks into such minerals as gneiss and mica- 
schist, are extensions of the metamorphic actions just 
described. They involve a more complete carrying out of the- 
principle of change, for the}^ are removed yet further from the 
normal condition of accumulated fragments rolled and worn 
by the action of water. 
