Prof. F. Lcewinson-Lessing — Origin of the Igneous Rocks. 249 



I mean the average chemical composition of the earth's solid crust as 

 a whole, and especially that of the igneous rocks. 



The problem of determining this average composition is very 

 attractive. It is well known that the most important contributions 

 to the solution of this problem have been given of late years by 

 F. W. Clarke, 1 H. S. Washington, and Yogt. Without denying the 

 value of their calculations and reasonings for elucidating the question 

 of the relative abundance of the different elements in the solid crust, 

 1 must nevertheless state that the method used by these and other 

 authors for calculating the average composition is in principle 

 erroneous, and consequently the result is arbitrary. The arithmetical 

 mean of hundreds or even thousands of analyses, when the relative 

 quantities of the different types of rocks are not taken into con- 

 sideration, has no serious value. Nevertheless the result is probably 

 very near to the real average composition, as will be subsequently 

 shown, because the figures given by Clarke 2 and several other authors 

 represent approximately the mean of the composition of granite and 

 gabbro (or basalt) ; and these two magmas are not only the most 

 widely spread, but they seem to enter into the composition of the crust 

 in nearly equal quantities. If we take, then, the mean only from the 

 figures for granite and gabbro (basalt) we get a result which in 

 essential features is nearly identical with the mean calculated from an 

 arbitrary number of separate analyses of all known types of igneous 

 rocks. Hut it is not this average magma, having nearly the composition 

 of a syenite, which I consider the primordial terrestrial magma, and 

 I do not think that all other rock-types such as granites, gabbros, etc., 

 are derivates, produced by differentiation of one fundamental magma. 

 On the contrary, as will be shown later, my hypothesis consists in 

 admitting that there are two and only two fundamental magmas — the 

 granitic and the gabbroidal ( = basaltic), and that the eruptive rocks 

 are derived from these two magmas by differentiation and assimilation. 



TABLE I. 



1 See, for instance, Clarke, " The Data of Geochemistry " : Bull. U.S. Geol. 

 Surv., No. 330, 1908. 



2 It must be noticed that in calculating the average composition of the whole 

 crust Clarke (ibid.) takes for the sediments and the eruptives certain coefficients 

 corresponding to their relative quantities in the solid crust. But why is the 

 same method not applied to the calculation of the average for the eruptive rocks ? 



