EVIDENCES FAVORING HYPOTHESIS OF ORIGIN 111 



Fourth. Mineral derived from organic matter in limestone ( ?) : 

 graphite in some nephelite syenites. 



In Clarke's "Data of Geochemistry'' a large number of experiments un 

 the origin of these minerals are described in abstract from the writings 

 of many observers.*' These artificial reproductions show instructive anal- 

 ogies to the processes which doubtless affect the crystallization of a lime- 

 stone-basalt syntectic. Particular reference may be made to the syn- 

 theses of cancrinite, nephelite, leucite, analcite, haiiynite, corundum, 

 melilite, scapolite (meionite), wollastonite, garnet, titanite, and graphite. 



The metamorphism of limestones and dolomites by intrusive masses 

 involves chemical changes which are somewhat similar to those taking 

 place in the syntectic of limestone and subalkaline magma. The produc- 

 tion of garnet rock, epidote rock, diopside rock, or amphibolite in con- 

 tact aureoles implies some desilication of the magma. It loses iron 

 oxides as well. The alkalies are normally transferred into the contact 

 rock to small degree, and are thus left concentrated in the magma. 



A few relevant experiments may be noted. Limestone flux precipi- 

 tates magnetite from furnace slag. Similarly, diopside and wollastonite 

 molecules are, after the lapse of sufficient time and at moderate super- 

 heat, segregated in special abundance at the bottom of glass furnaces, the 

 overlying layers of the melt being richer in the alkaline silicate. On the 

 other hand, the writer is not acquainted with any case where the positive 

 concentration of alkalies in a subalkaline magma through the action of 

 limestone fluxing has been carefully studied in laboratory or commercial 

 plant. 



Possible Objections to the Hypothesis 



At present this suggestion as to the origin of the alkaline rocks ob- 

 viously suffers from incompleteness on the physico-chemical side. How- 

 ever, enough is already known to assure any future investigator that he 

 will obtain valuable results on fashioning his experiments along the lines 

 of the present hypothesis; the geological facts seem to show that those 

 results will favor the general idea of limestone control, or at least carbon 

 dioxide control, in the development of alkaline rock magmas. 



The hypothesis as above outlined is a special case in the general syn- 

 tectic-diflerentiation theory of igneous rocks and may prompt the objec- 

 tions which have been directed against the theory.' There are plenty of 

 field examples showing that the magmatic heat of basalt is ample to fuse 



«F. W. Clarke: Bulletin 330, U. S. Geological Survey, 1908. 

 T R, A. Daly : American Journal of Science, vol. 26, 1908, p. 49. 



