DUE0CHER S COMPARATIVE PETROLOGY. 
159 
Essay on Comparative Petrology ; or, Researches on the Chemical 
AND MlNERALOGICAL COMPOSITION OF IGNEOUS RoCKS J ON THE PHE¬ 
NOMENA of their Eruption ; and on their Classification. By M. 
J. Durocher, Mining Engineer and Professor of the Eacnlty of Science 
at Rennes. 
[Translated from the “ Annales des Mines,” vol. xi., 1857.] 
Object of the Memoir. —This Essay gives the general results of a series 
of researches in which I have been engaged for a long time on the igneous 
rocks, and which are not yet completed ; some of them, however, have 
already appeared in various journals, viz., “ Toy ages en Scandanavie,” 
“ Comptes rendus de l’Academie des Sciences” (tom. xx. p. 1277; 
tom. xxiii. p. 978 ; tom. xxv. p. 208; and tom. xliv. pp. 325, 459, 
&c., &c.); the “ Bulletin de la Societe Geologique” (2nd ser. tom. iv. 
pp. 409, 1018; and tom. vii. p. 276); and the Memoirs of the same 
Society (2nd ser. tom. vi. first part). 
Eor many years the eruptive rocks have been the subject of impor¬ 
tant investigations, both in France and Germany; hut these investiga¬ 
tions have only given rise to works of detail, of which the object is to 
determine the mineral species found in the rocks. The present Essay is 
a “ Memoir on Comparative Petrology,” and ought to be considered as 
an attempt at a general synthesis of the pyrogenous rocks, considered in 
the fourfold point of view,—of their Chemical constitution, their Mine- 
ralogical composition, their Eruption, and their Classification, 
Part X. 
reduction of all the igneous rocks to two magmas. 
Office of Silicon in the Mineral Kingdom. —In the mineral kingdom Si¬ 
licon performs a part analogous to that of carbon in the organic kingdom; 
and in its behaviour, as a polybasic acid, silica unites with the oxides in 
various proportions, and thus gives rise to numerous combinations. Most of 
the mineral species which thence result, and especially those which enter 
into the composition of the crystalline rocks, arise from combinations of 
elements which are always the same, and whose total proportions in the 
mass containing them vary only within narrow limits. In seeking the 
mineral silicates, whose aggregation constitutes rocks, it is not necessary 
that each association forming a distinct rock mineralogically should also 
correspond to a special chemical composition of the Magma which pro¬ 
duced it. This appears to me to constitute one of the most important 
views of the study of rocks, and the researches which I have undertaken 
on this subject have led me to results remarkable for the simplicity which 
they introduce into the history of igneous formations, and which, besides 
being founded on experimental data, appear to me to agree perfectly 
with geological observations. 
