SYSTEM A TIC PETROGRA PHY 477 



earlier one 1 upon the summary given by F. Loewinson-Lessing. 2 



Schrockenstein's view of igneous rocks will, on account of 

 its fantastic imaginings, impress many a reader as belonging 

 rather to the eighteenth, or a still earlier, century, than to the 

 close of the nineteenth ; and, although his propositions are of 

 no consequence to petrography, the fact that they have been put 

 forth at all, in the very last decade of the period in review, has 

 a certain melancholy interest. 



Schrockenstein considers the original crust of the earth to 

 have been a silicate of alumina, probably with excess of silica. 

 This simple primary magma is conceived to have been first ren- 

 dered impure by meteoric showers, introducing lime, magnesia, 

 and iron. At a later period the alkalies and water were precipi- 

 tated from the atmosphere. The alkalies are considered as of 

 very subordinate (" nebensachlich ") importance and the chemical 

 problem of rocks, as the author views it, is to compare the 

 relative amounts of the original alumina silicate and the meteoric 

 impurities. That is to say, the author proposes classes according 

 to the degree of adulteration of the original magma and orders 

 according to the character of the adultera?it. 



The method followed by Schrockenstein in comparing analyses 

 of silicate rocks appears to be somewhat as follows : 



First, magnetite is calculated out, as an extraneous substance, 

 whenever the analysis is sufficiently modern, through determi- 

 nations of both ferric and ferrous oxides, to give a basis for such 

 calculation. When the analysis is inadequate and the iron is 

 lumped under one or the other oxide, the result is accepted by 

 the author and Fe 2 3 is supposed to replace alumina or FeO is 

 added to MgO and CaO. Not until magnetite is deducted does 

 Schrockenstein consider that the real rock is under discussion. 

 Inasmuch as he states that after deducting magnetite there is 

 either no iron left or but one oxide appears, it is evident that the 



1 Ausfluge auf das Feld der Geologie," Geologisch-chemische Studie der Silicat- 

 gesteine, II Auflage, Wien, 1886. 



2 "Studien ueber die Eruptivgesteine," Compte-Rendu, VII Cong. Geo/. Internat., 

 1899, p. 196. 



