METAMORPHISM. 
41 
little cohesion. The marble^, however, is changed, and partially 
crystallised ; it has a peculiar and definite texture ; it is 
extremely compact, and of uniform colour and hardness. It is 
converted into a substance apparently distinct and with dif- 
ferent properties. It is metamorphosed. 
So also with regard to clay and slate. The latter is precisely 
similar to the former in composition, but the difference between 
them is in some respects enormous. For all practical purposes 
they are as different as limestone and quartz, but still the slate 
is nothing more than a metamorphosed clay. 
There is nothing mysterious in metamorphosis; it is a change 
wrought by time and circumstance, and in zoology it repre- 
sents a development of inorganic matter corresponding to 
well-known developments of organic matter. But although 
not miraculous, it is surrounded with interest and involves 
many difficulties. The analogy with the phenomena of life is 
not to- be assumed without some knowledge of the circum- 
stances of the case. 
Every one who has even a superficial knowledge of the main 
outlines of geology is aware of the existence of certain rocks 
called slates and schists, which are so different from the com- 
mon stones and rocks deposited from water, that they seem at 
first to have no relation with them. In all geological books 
these are called metamorphic rocks, and they well deserve the 
name. These are but examples of a particular stage of meta- 
morphic action. The changes leading to the formation of 
slates and marbles require to be foUowed out from the com- 
mencement, if we would understand what they are, and what 
they lead to. 
It is not easy to acquire a knowledge of the mode in which 
Nature acts on a large scale in the accumulation of the 
deposits that will ultimately appear at the earWs surface, ^s 
the limestones, sandstones, and clays, of some yet unborn 
continent. That all such infant and growing land is now under 
water; that a large part — almost the whole — is of necessity 
bmfied not only by water, but by a constantly thickening crust 
of similar material, each layer more recent than the rest, and 
therefore, still less affected by any change, there cannot be a 
doubt. Setting aside all theory, — all attempt at investigating 
doubtful jDroblems and questionable phenomena, — who can con- 
sider for a moment the state of things on the earth without 
coming to this conclusion ? The two ends of the great natural 
scale are manifest enough. We trace the recently heaped 
mud and stones at the bottom of shallow water, and we can 
examine the rocks from which they have been removed. The 
derivation of river detritus and sea mud from the adjacent soil, 
and from animals living on land or in water, is not difficult to 
