44 



general law. In 1851 BUXSEX pointed out that (1) the varying composition of 

 a large series of igneous rocks which he had examined from different parts 

 of the world could be expressed with a very considerable amount of accuracy 

 on the assumption that they were due to the mixture, in varying proportions, 

 of two distinct magmas, which he designated by the terms norm al-trachy tic 

 and normal-pyroxenic. STREXG, TKIHOLET and others immediately applied 

 the ideas of BUXSEX to the igneous products of the districts with which 

 they were severally acquainted. 



In 1857 DUROCHER published his " Essai de Petrologie Comparee," (2) in 

 which he propounded the theory that the igneous products of all ages have 

 been derived from " two magmas which co-exist beneath the solid crust of the 

 globe and occupy there each a definite position." The terms acid and basic 

 were given to these two magmas, and their average compositions were sup- 

 posed to be as follows : 



ACID. BASIC. 



Si0 2 71-0 51-5 



A1 2 3 16-0 10-0 



FeO 2-o 18-0 



CaO ]-0 8-0 



MgO 1-0 6-0 



K 2 O 4-5 1-0 



Na 2 O 2-5 3-0 



Sp. Gr. of crystalline rook ... 2'65 ... ... 2'95 



Sp. Gr. of glass 2-40 272 



The majority of the igneous rocks, according to the author, belong to 

 one or other of these types. Rocks of intermediate composition, such 

 as the andesites and porphyrites, are supposed to be due to the intermixture 

 of the two magmas at the zone of contact. Certain rocks of exceptional 

 composition, such as the phonolitcs, are regarded as due to a process of 

 liquation analogous to that which goes on under certain circumstances in 

 molten metallic mixtures, and which is utilized, for example, in the Pattinson 

 process for separating silver and lead. 



The theory of DUROCHER, in so far as it is an expression of the facts 

 known with reference to the composition of igneous rocks, is substantially 

 identical with that of BUXSEN, and may be regarded simply as an extension 

 of the latter with slight modifications ; but in so far as it deals with the 

 physical condition of the planet it is peculiar. We are not, however, con- 

 cerned with the latter aspect of the theory. 



YON COTTA (3) adopts a division of the igneous rocks into two principal 

 groups which agree substantially with those of BUXSEX and DUROCHER. He 



(1) P. A. LXXXIIL, p. 197. 



(2) A translation of this Essay is given in D/. HATJGHTON'S Manual of Geology. 



(3) Rocks Classified and Described. Translation by Lawrence. 1866. 



