PETROLOGICAL ABSTRACTS AND REVIEWS 
ALBERT JOHANNSEN 
LeitH, C. K. and MEap, W. J. Metamorphic Geology. Henry 
Holt & Co., New York, 1915. Pp. xxiv+337, figs. 35, pls. 16. 
This is one of the most valuable of recent petrologic textbooks. 
The experience of the authors in the interpretation of metamorphic 
processes would make this work of great value even though there were 
many books on the subject; since this is actually the only general text- 
book, it is doubly valuable and welcome. 
‘Under the term rock-metamorphism the authors include not only 
the changes involved in the formation. of rocks commonly called meta- 
morphic, but also all mineralogical, chemical, and physical changes 
which have taken place in rocks subsequent to their primary crystalli- 
zation from the magma, thus including in the definition cementation 
and rock-weathering. In other words, the metamorphic processes here 
included are both the destructive processes of katamorphism and the 
reconstructive processes of anamorphism, together forming the meta- 
morphic cycle. 
The writers take up first the katamorphism of acid igneous rocks, 
katamorphism being used “to cover all alterations of a disintegrating 
or decomposing nature, whether accomplished by weathering or by 
thermal solutions, whether at the surface or below.” The most impor- 
tant phase of katamorphism is weathering, or the alteration of surface 
rocks by the agencies of the atmosphere and hydrosphere. By this 
process the rocks decompose and disintegrate, and part of the constitu- 
ents are carried away in solution, while the residue forms a porous mass 
which in many cases retains the texture of the original rock. Clay, 
sand, carbonate, and salt in aqueous solution are the end-products of 
katamorphism. Chemically the changes are chiefly hydration, car- 
bonation, oxidation, and desilication; mineralogically there is a destruc- 
tion of some minerals and a simplification of others, the minerals in many 
cases passing through intermediate forms before reaching the simple 
end-products. Another change produced is one of volume, there being 
an increase by the formation of pore space, by the addition of water, 
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