SUMMARY OF PHYSICO-CHEMICAL PRINCIPLES. Dill 
hence all metamorphism is partly chemical metamorphism. When it is 
realized that in all the varieties of metamorphism mentioned, chemical 
action, heat, and dynamic action enter as factors, only i in alierent de- 
grees, and nia it is remembered that water is the universal agent which 
is present and active wherever metamorphism occurs, it 1s ccliteriilent 
that the classifications ordinarily given are not satisfactory. 
A critical examination of the classifications of metamorphism shows 
that the kinds of metamorphism recognized in the text-books are based 
upon the idea that the particular force or agent mentioned is dominant 
in the production of the particular metamorphism. However, they in- 
volve different factors, not belonging to the same category. For instance, 
thermo-metamorphism refers to heat; contact metamorphism refers to 
the contiguity of an igneous rock; hydro-metamorphism refers to the 
presence of water. As a matter of fact, all of the different kinds of meta- 
morphism are related in the most intricate manner, and certain kinds of 
metamorphism which have been called thermo-metamorphism might 
just as well be called hydro-metamorphism. 
UNDERGROUND FLOWAGE OF WATER 
Underground water, the agent of metamorphism, needs to be consid- 
ered from two points of view—(1) its movement and (2) its work. 
(1) MOVEMENT OF UNDERGROUND WATER 
The movements of underground waters are dependent upon (a) head, 
(b) the underground openings, and (c) viscosity. 
(a) The flowage of underground waters is fundamentally caused by 
head. Head is due chiefly to the fact that the water, entering the ground 
at a certain level, after a longer or shorter underground course, issues at 
a lower level. Head also depends somewhat upon temperature. In so 
far as the water is warmer at its point of issuance than it was when it 
joined the zone of saturation, this is favorable to circulation, and gives 
an effect in the same direction as difference of elevation. This is due to 
the fact that the density of water varies inversely with the temperature. 
That the difference in density due to difference in temperature is suffi- 
cient to produce rapid circulation in pipes which are non-capillary is 
shown by the use of the principle in the hot-water system of heating of 
buildings. In such regions as the Yellowstone Park, and in fact any of 
the regions where igneous rocks have been lately intruded, the difference 
in temperature in the descending and ascending columns may be an 
important influence in the circulation. Underground circulation may 
be caused also by difference in temperature of the descending and as- 
