REVIEWS 
Igneous Rocks. Vol. I, Composition, Texture and Classification. 
By JosrpH P. Ippincs. New York: John Wiley & Sons, 1909. 
The application of modern ideas of physics and chemistry to the study 
of the rocks may be seen, more and more, in receat publications, and the 
results of these newer investigations*make it necessary to look at the 
problems of petrology from an entirely new point of view. It is rather 
remarkable, and an indication of the need of a systematic presentation of 
the subject, that within the past few months there have appeared two 
volumes, following essentially similar plans, and differing entirely from 
previous works on petrology, namely: Professor Harker’s Natural History 
of Igneous Rocks, and Professor Iddings’ Igneous Rocks. 
Professor Iddings introduces the subject with a consideration of the 
chemical composition of igneous rocks as a whole without regard to the 
mineral constituents, and he describes the various graphical methods used 
by petrographers to express the proportions of chemical elements in these 
rocks. 
The magma, solidifying as a rock, takes, in general, the form of a 
mass of crystals with definite chemical and physical characters, and more 
or less glass. The relations to each other of the so-called pyrogenetic, or 
primary, minerals are shown by the presence in them of elements which 
occur together in the same groups in Mendeléeff’s table. Consequently 
many of the minerals in a given rock are characterized by their content 
of different amounts of the same element. 
The principles of physics and chemistry applicable to rock magmas 
are fully discussed. Rock magmas are now believed by everyone to be, 
not simple melts, but solutions of minerals at high temperatures which 
may act like solutions of other compounds at lower temperatures. In the 
latter, the reactions taking place, the processes of solidification which con- 
vert them into glasses or crystals, and the physical characters and molecu- 
lar constitution of the liquid solutions are more easily understood than 
they are in rock magmas at high temperatures and pressures. In order 
to study properly the facts of physical chemistry, there are discussed the 
kinetic theory of gases, liquids, and solids, the melting points of rock 
minerals, and the physical characters and chemical reactions of solutions. 
After this preliminary statement of principles, the discussion of the chemi- 
756 
