PETROLOGICAL ABSTRACTS AND REVIEWS 279 
similarity of the phenomena exhibited at the emery mines with those 
shown on the borders is thought to indicate that the ore owes its origin 
to the inclusion and assimilation of schistose xenoliths, producing a 
local supersaturation of alumina which resulted in the deposition of 
corundum. 
AUTHOR’S ABSTRACT 
ScHWARTZ, E. H. L. “What Is a Metamorphic Rock?” Geol. 
Mag., VIII (1911), 356-61. 
In propounding this apparently academic query, the writer is actuated 
by the appearance of certain taxonomic inconsistencies in the latest 
treatise on metamorphism—Grubenmann’s Die kristallinen Schiefer— 
which he believes would have been obviated by a more precise definition 
of the term. Van Hise defines a metamorphic rock as one that has been 
altered; but it is tacitly understood that a sediment does become meta- 
morphic through induration, for example, nor will the development of 
pegmatitic structure remove a granite from the igneous class. Starting 
with the conception of the three zones of metamorphism, marked by 
increasing pressure and by the increasing molecular volumes of the rocks 
formed, Professor Schwartz notes the fact that the molecular volume of a 
true igneous rock will be still higher. Now solvent water is the agent 
active in forming the characteristic minerals and structures of igneous 
rocks, and this leads to an essential difference between them and the 
metamorphics. In the former the pressure is so great that the solid 
particles move freely into the solvent, allowing the chemical affinities 
of the molecules to satisfy themselves, while in the latter only the borders 
of the grains are rendered fluid, the chemical affinities are restricted, and 
the law of least molecular volume comes into play. Thus a meta- 
morphic rock would be one in which the internal or molecular pressure 
had been less than the external or dynamic, so that certain peculiar 
minerals have resulted. Some of these, such as epidote, lose their crystal- 
line form long before fusion, indicating a probable expansion of the mole- 
cule, under conditions which allow the chemical affinities full play. 
These minerals when found in a rock may therefore be accepted as criteria 
of its real nature. 
This paper does not arrive at any very satisfactory conclusion, from 
a practical standpoint; but it must be remembered that it is primarily 
a protest against the want of definitiveness in the terms ordinarily 
employed, and that the author’s recourse to physical chemistry is both 
tentative and reluctant. 
G. S. ROGERS 
