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1893,] Mineralogy and Petrography. 41 
of the differentiation of a homogeneous magma. Scrope’s notion was 
a crude one, but it has been built upon little by little until it has, in 
the hands of Mr. Iddings, been placed upon a footing secure enough 
to warrant its being thoroughly tested by observation and experimen- 
tation. The author points out the evidences of the close relationships 
exhibited by the rocks emanating from a volcanic center and their 
differences from similar groups from other centers, and then takes up 
the question of the differentiation of molten magmas. He brings for- 
ward geological and chemical evidences of the fact of differentiation, 
and explains the act upon Soret’s principle that in a solution whose 
parts are at different temperatures there will be a concentration of the 
salt in the colder parts. Lagorio has shown that rock magmas are 
solutions, and Iddings believes they are solutions of the chemical ele- 
ments or of their oxides. Consequently, after differentiation has taken 
place and cooling sets in, different mi ] fi d ing to laws 
that depend upon the proportions of the oxides occurring in the differ- 
entiated portions. This is apparently contradictory to the view of 
Rosenbusch,* who regards rocks as having originated in the differen- 
tiation of a magma, but of a magma which is a solution of silicate salts 
in a silicate solvent. As a result of the condition of affairs suggested 
by Iddings the first eruption from a volcanic center would naturally 
possess a composition intermediate between those of succeeding erup- 
tions. As a fact the author states that the sequence is usually a rock 
of intermediate composition, followed by less siliceous and more silic- 
eous ones, to those very basic and very acid. The last eruptions are 
of very exceptional character. These will occur in small quantity 
only, and will be first eroded from the surface. Consequently these 
forms will be found principally in dykes. They are the forms to which 
Rosenbusch has given the group name “Ganggesteine.” These rocks, 
according to Iddings, have their equivalents among volcanic flows, but 
the association of minerals in them is different. It is simply their 
structure, therefore, that characterizes the dyke rocks. They have 
originated in the same manner as have other eruptives, and conse- 
quently are not essentially different from them. The author’s views 
are developed carefully and at considerable length. They will 
undoubtedly serve to turn the attention of petrographers to a subject 
that has lain neglected long enough—the comparative study of rocks 
of single geological provinces. The paper will well repay very careful 
reading by all petrographers and theoretical geologists, who should be 
SAMERICAN Natura.ist, Nov., 1890, p. 1071. 
