Prof. F. Lcewinson-Lcssing — Origin of the Igneous Rocks. 253 



Rosenbusch notes that the monzonitic and essexitic magmas are 

 subject to large differentiation processes; this assertion, however, 

 cannot be considered as correct, because the monzonites are one of 

 the products of the largely differentiated gabbro-syenitic magma, but 

 are not sensibly subject to differentiation. 



Our conclusion is then the following : — Not one of the magmas, 

 syenitic, monzonitic, or dioritic, can be considered as the primordial 

 magma, from which were derived and are still derived all eruptive 

 rocks. And not nearer to the truth are those who urge that the 

 primordial magma is either a granitic or a basaltic one. Thus Mennell * 

 criticizes the calculations of Clarke and others, and is quite right iu 

 objecting to their calculations, as they ought to have taken into 

 consideration the relative quantities of the different types of igneous 

 rocks. His own calculations for apart of Rhodesia lead him to support 

 the view of a granitic magma ; presuming that the granitic rocks 

 equally predominate in all other countries, he is logically induced to 

 consider the granitic magma as representing the primordial terrestrial 

 magma. But the predominance of granitic rocks is certainly only 

 apparent. For Sweden we have calculations which demonstrate that 

 the granitic and the basic rocks are represented in nearly equal 

 quantities ; and such calculations would certainly give the same result 

 for many other countries, where granites seem to predominate. And 

 if in such parts of the globe as Finland the granitic rocks really or 

 apparently predominate, there are other districts, as the TJral 

 Mountains, where the predominance is certainly on the side of the 

 different members of the basic magma, the gabbro - peridotitic- 

 pyroxenitic magma. 



What has been said of Mennell's hypothesis, is in full measure 

 applicable to the basaltic magma, the antipodes of granite. Daly, who, 

 as has been mentioned above, considers the terrestrial average to be 

 a ' dioritic ' magma, formerly expressed another point of view. In 

 1903 2 he regarded as the universal earth magma the basaltic 

 (gabbroidal) magma, and pointed out that the other eruptive rocks 

 are derivates, produced by abyssal assimilation and magmatic 

 differentiation. This opinion was supported by the following facts : — 

 The fissure-eruptions and the other cases of eruptions on a great scale 

 are represented by the basalts ; the basalts and andesites have a much 

 wider distribution than other volcanic rocks ; the last products of 

 eruption (when the primordial magma is not further influenced by 

 assimilation) are basaltic rocks (p. 110). 



Dutton 3 has expressed similar ideas ; he also considers the basalts 

 as the ' primordial matter ', and infers that the other types of volcanic 

 rocks are the result produced by a refusion of sedimentary formations. 

 But are the basalts and andesites really predominant amongst the 

 volcanic formations? A definite and exact answer to this question 



1 F. P. Mennell, " The Average Composition of the Igneous Bocks," Geol. 

 Mag., 1904, p. 263; "The Constitution of the Igneous Eocks," ibid., May, 

 1909, p. 212. 



2 E. Daly, " The Geology of the Ascutney Mountains, Vermont " : Bull. U.S. 

 Geol. Surv., No. 209, 1903. 



3 Dutton, The High Plateaux of Utah, 1880, p. 125 (cited after Daly). 



