356 C..K. LEITH AND W.s. MEAD 
at a maximum and the ‘“‘zone of anamorphism”’ for a deeper region 
where the reverse changes are at a maximum; thus emphasizing 
influence of depth on metamorphic changes. ‘These terms are 
extremely useful in the general sense in which Van Hise used them. 
It is difficult, however, to use them in the inductive study of rock 
changes, for so may other factors enter that at any depth the 
changes may be in opposite directions for different kinds of rocks 
or for the same rock at different times. The conception of the 
metamorphic cycle is better adapted to an inductive study of the 
rocks, for it relates merely to the succession of changes in the rock 
without reference to depth. 
INFERENCES AS TO SIGNIFICANCE OF CYCLE 
It is not enough merely to have established the existence of 
these cycles. We are interested in knowing the total energy 
changes involved, what keeps the cycle going, whether the cycle 
is closed, the net results in distribution of rock substances. 
ENERGY CHANGES INVOLVED IN THE CYCLE 
Much remains to be learned about the energy changes in com- 
mon rock alterations, but enough is quantitatively known to enable 
us to state, with some confidence, that the changes of the meta- 
morphic cycle result in a permanent net loss of energy from the 
system in the form of heat. It may be assumed from this and 
from a priori reasoning that it is this running down of energy which 
keeps the cycle going. In the igneous rocks energy is locked up in 
the molecular combination of minerals which under katamorphism 
is liberated as heat and much of it permanently lost. As this 
change involves expansion, it is constantly opposed by gravity, 
while in the anamorphic phase of the cycle gravity controls, 
leading to a diminution of volume. We may suppose that at 
any given depth some sort of energy equilibrium exists between 
pressure and the energy locked up in molecular combinations. So 
far as a rock is not adapted to this state of equilibrium, changes 
will goon. A granite formed under conditions of high temperature 
and high pressure when brought to the surface finds itself under 
conditions of low temperature and low pressure. The expansive 
