REVIEWS 
The Natural History of Igneous Rocks. By ALFRED HARKER. New 
York: Macmillan, 1909. 
The scope of Harker’s Natural History of Igneous Rocks may be 
indicated by the titles of the chapters which are as follows: (1) “‘Igneous 
Action in Relation to Geology;” (i1) ‘‘Vulcanicity;” (iil) ‘‘Igneous Intru- 
sion;” (iv) Petrographical Provinces;” (v) ‘‘Mutual Relations of Asso- 
ciated Igneous Rocks;” (vi) ‘‘Igneous Rocks and Their Constituents;” 
(vii) ‘‘Rock-Magmas;” (viii) ‘“‘Crystallization of Rock-Magmas;” (ix) 
“‘Supersaturation and Deferred Crystallization;” (x) ‘‘Isomorphism and 
Mixed Crystals;” (xi) ‘Structures of Igneous Rocks;’’ (xii) “‘ Mineralisers 
and Pneumatolysis;”’ (xiii) ‘‘Magmatic Differentiation;” (xiv) ‘‘Hybrid- 
ism in Igneous Rocks;” (xv) ‘Classification of Igneous Rocks.” 
The author emphasizes the correlation between the general geologic 
history of a given region and the igneous activity in the same region, which 
he makes on the general basis of the idea that igneous action is a result 
of crustal movements, and the further idea that these movements produce 
magmatic differentiation over continental areas; thus leading to magmas 
of different composition in regions affected by different kinds of crustal 
movements. It becomes apparent, therefore, that the author accepts the 
idea of differentiation, and applies it as a means of explaining the distri- 
bution of igneous rocks in all parts of the earth’s crust, whether those parts 
be treated in detail or on the largest scale. It follows as a natural con- 
sequence that he explains petrographic provinces as due to differentiation 
over large areas, while the different types of a given province are explained 
as due to differentiation within that province. 
On the other hand, Harker accepts Vogt’s conclusion that silicates, 
silica, and water are miscible in all proportions at the temperatures of 
magmas, and that consequently there can be no liquation or spontaneous 
division of a single magma into different parts while the whole is still 
liquid; except insofar as such a division may be explained by differences 
in pressure, temperature, or density. He further concludes that the 
liquation due to pressure and density would be negligible in amount, while 
he thinks the differential action of gravity would perhaps suffice to con- 
centrate the denser constituents in the lower portion of a large magma. 
With this possible exception he attributes differentiation to the concurrent 
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