18 The American Geologist. January, 1897 
termediate between fusion and ordinary atmospheric action. 
The limits are not sliarply defined, so metamorphism grades 
below into igneous action, and above into atniosj)heric action 
or weathering. Metamorphic rocks may be further metamor- 
phosed so that all rocks, sedimentary, igneous and metamor- 
phic, are subject to metamorphism. The agencies at work in 
this great process are both physical and chemical and they 
are classified according to the preponderating influence. If 
the temperature and pressure are low the action is due mainly 
to water, producing hydro-metamorphism., resembling very 
closely weathering. If the temperature is high and pressure 
is low, and mineralizers — gases whose presence facilitates fu- 
sibility — are present the action is described as suhlimation- 
metamorphism ; or, if water alone be present, the action is 
described as ther)uo-melamo7'phisin. Static-metatnorphism in- 
cludes those changes where pressure is mainly active and 
where motion is absent. If motion is present the changes 
come under the division of dynaniic-metumorphism. All of 
these alterations take place without any material change in 
bulk composition. Static metamorphism, though not accepted 
by many geologists, has able defenders in such men as Hall, 
Judd, Spring. Dynamical metamorphism has been firmly es- 
tablished by the classic works of Heim, Lehman and Balzer. 
Metamorphism may be produced by the presence of some 
metamorphosing agent, and it is then termed contact meta- 
itiorphism. The alteration in surrounding rocks may extend 
over a distance from a small fraction of an inch up to 4,000 
feet, as seen in the Pyrenees. The nature of contact meta- 
morphism depends on the duration of the action, the depth 
at which the alteration takes place, whether deep enough to 
prevent escape of the vapors and moisture or not: and on the 
nature of the metamorphosing agent whether this is a gran- 
ite, diabase or other rock ; and also on the nature of the rock 
altered, whether crystalline schists, carbonaceous rocks, sand- 
stones or igneous rocks. The nature further depends on the 
structure of the rocks, whether foliated or not, as demonstra- 
ted in Brittany by Barrois. 
Kosenbusch, after a careful examination of analyses, con- 
cludes that there is no change in the bulk composition of the 
altered rocks, though Michel Levy insists that there is always 
a very considerable addition of substance. 
