METAMORPHISM 367 
phous carbonate of lime, either in the form of large 
fragments of shells, or else smaller particles of the 
caleareous portions of animals. The first change that 
is noticed after consolidation, is the growth of crystal- 
line bits of calcite, whose cleavage surfaces sparkle as 
the rock is turned in the light. This may proceed 
until a sugary white marble is formed; and in the 
large masses of this altered or metamorphosed lime- 
stone there is seen a kind of banding, like that so 
often noticed in the bluish-white marbles. 
Or the rock which is changing may be a clay. If 
so, the first difference that is noticed is transforma- 
tion to a dense, hard rock. This may then be altered 
to a slate, which splits easily into many different lay- 
ers. This slaty structure is due to the presence of 
innumerable micaceous flakes of minerals. All these 
are arranged with their flat faces parallel, so that the 
rock splits in this direction in a manner somewhat 
analogous to the splitting of a sheet of mica. These 
newly formed minerals may not admit of detection 
with the eye, though their presence is shown by the 
shiny surfaces of the slate; but the microscope will 
reveal their presence. Here then, this rock has under- 
- gone another kind of change. In the limestone it was 
chiefly an alteration of amorphous to crystalline car- 
bonate of lime; but in the slate, a new mineral has 
grown out of the complex elements of the clay. 
