688 



L. V. PIRSSON 



while the larger bodies of magma would tend to form theralites, 

 ijolites, etc., and the surface lavas would appear as nephelite teph- 

 rites, basanites, basalts, nephelinites, etc. 



As a corollary of this it would follow that all of these rocks 

 should possess a general similarity of chemical composition, 

 which in fact they do, as may be seen from the following table 

 of analyses : 



I Monchiquite, Brazil (Hunter and Rosenbusch op. cit.). Hunter anal, 

 n Monchiquite, Brazil (Hunter and Rosenbusch op. cit.), P. Jannasch anal. 



III Monchiquite, Magnet Cove, Ark. (Williams Igneous Rocks, Ark., 1890, p. 295), 

 W. A. Noyes anal. 



IV Monchiquite-Camptonite, Bohemia (Hibsch Tscher. Mitt. XIV, 1894, p. loi), F. 

 Hanusch anal. 



V Theralite, Crazy Mts., Montana (Wolff Petrog. Crazy Mts., 1885), J. E. Wolff 

 anal. 

 VI Theralite, Crazy Mts., Montana (Wolff Petrog. Crazy Mts., 1885), A. M. Comey 



anal. 

 VII Nephelite tephrite, Bohemia (Hibsch op. cit., p. 109), F. Pfohl anal. 

 VIII Nephelite basalt, Lobauer Berg, Heidepriem (Zirkel Petrog., 2d ed., Vol. III. 

 P- 37)- 

 IX Nephelinite, Laach., Eifel, vom Rath (Zirkel Petrog., 2d ed., Vol. Ill, p. 61). 



The list might be greatly extended but the above are suffi- 

 cient to show that the magmas producing these rocks have cer- 

 tain chemical characteristics in common, low silica, moderate 

 alumina and alkalies with soda predominating over potash, and 

 high lime, and iron, and high to moderate magnesia. 



In this connection the author cannot refrain from pausing a 

 moment to call the attention of petrographers to the fact, appar- 

 ently not often recognized, that analyses of basic rocks, rich in 



