SEDIMENTARY AND METAMORPHIC ROCKS 103 
A final change is one in which even the original 
condition is hidden. The last stage of metamorphism 
before actual melting produces gneiss (Fig. 46), which 
is much like a granite, excepting that it has its 
minerals more or less perfectly banded, while granite 
is massive and without layers. In the two rocks, the 
minerals are very often the same in kind, and in 
general form. There are many varieties of gneiss, 
whose names need not be introduced here. 
Complexity of Metamorphism. — The metamorphic 
rocks present a large number of complex and difficult 
phenomena, which we are only just beginning to under- 
stand by means of careful study with the microscope. 
It was once thought that they represented the original 
crust of the earth, and possibly some of them do; 
but careful study has shown that many are merely 
altered conditions of other rocks. 
In some places, the changes have been so well de- 
termined, that the gneiss can be shown to be an altered 
form of granite, or of a sedimentary or some other 
rock. In these cases, the changes are many and 
varied, including the action of heat, pressure, the 
presence of water, and the introduction of numerous 
complex chemical reactions. 
Characteristics of Metamorphic Rocks. — Leaving out 
of consideration those rather simple forms, such as 
the quartzite, coal, and slate, whose origin is easily 
