THE ORIGIN OF IGNEOUS ROCKS. 
BY 
Joseph Paxson Iddings. 
[Read before the Society May 7,1892, and published by permission of 
the Director of the U. S. Geological Survey.] 
CONTENTS. 
Page. 
Introduction.......... 90 
Historical Review—Scrope, Darwin, Dana, Bunsen, Yon Wal- 
tershausen, Lyell, Jukes, Durocher, Yon Cotta, Roth, Yon 
Richthofen, King, Dutton, Rosenbusch, Brogger, Yogt, Dakyn, 
and Teall.... 91 
Consanguinity of igneous rocks..... 128 
Mineralogical evidence of consanguinity. 130 
Chemical evidence of consanguinity. -.. 135 
Geological evidence of consanguinity. 143 
Differentiation of molten magmas. 144 
Geological evidence. 144 
Chemical evidence. 149 
Character of differentiation. 151 
Absence of stoichiometric proportions. 152 
Chemical independence of the essential oxides. 154 
Cause of differentiation. 156 
Molten magmas as solutions. 156 
Soret’s principle. 158 
Chemical equilibrium affected by pressure. 159 
Process of differentiation. 160 
Results of differentiation. 164 
Mineral composition of rocks. 164 
Extreme results of differentiation. 167 
Exceptional dikes and flows of the Absaroka range. 169 
“ Ganggesteine ” in general. 172 
Localization of volcanic phenomena. 178 
Irregular order of succession.. 178 
Physical considerations. 179 
Fissures as reservoirs of magma. 181 
Dynamical considerations. 181 
Localized magmas. 183 
Resume. 186 
Rock Analyses. 198 
15—Bull Phil. Soc., Wash., Vol. 12. (89) 
