102 IOWA ACADEMY OF SCIENCES. 



established: (1) The ultra-basic rocks, derived from a 

 monosilicate magma; (2) basic rocks, which had a bisili- 

 cate magma; (8) neutral rocks, with a magma which was 

 bisilicate or normal, and (4) acid rocks, in which the magma 

 was polysilicate. These groups are subdivided into four- 

 teen sub-groups and thirty-four families. 



In order to find the proper systematic position of an 

 eruptive rock from the fundamental viewpoint of the pro- 

 posed classification, four factors are considered: (1) The 

 relation of the oxygen in the silica and that in all the other 

 oxides taken together, giving what is termed the coefficient 

 of acidity; (2) the chemical composition, which gives for 

 each type a distinct formula; (3) the relations between the 

 two groups of oxides according to their molecular propor- 

 tions, and (4) the relations of the soda and potash in the 

 alkaline rocks. This consideration of the principles of 

 classification leads to the proofs of the distinct phases of 

 fundamental magmas. 



Discussion of the differentiation of rock magmas has an 

 unusual interest. The Russian author calls special atten- 

 tion to the principle of Soret, the action of super-saturated 

 solutions, the effect of gravity, the principle of maximum 

 work as proposed by Berthelot, and the reactions of mixed 

 liquids, as operating in the separation of magmas. 



Three distinct kinds of magmatic differentiation are 

 recognized. They are: Static differentiation, taking place 

 in the depths of the earth; differentiation by cooling dur- 

 ing ascent to the surface; and crystalline differentiation. 

 Specific gravity, pressure, and temperature are the chief 

 factors governing the course of the static kind; while 

 chemical affinities come into play in large measure only 

 in crystalline separation. 



The role of inclusions of foreign rocks, which has so 

 long been such an unsatisfactory subject to petrographers, 

 is explained on the idea that it is only that portion of the 

 magma yet undifferentiated which affects the introduced 

 rocks. After thorough assimilation of limestone for 

 example, a separation of the modified magma takes place. 



