llO R. A. DALY ORIGIN OP THE ALKALINE" ROCKS 



inents have been made on such a solution. It appears possible that, in 

 the absence of uncombined silica, the carbon dioxide would form com- 

 pounds with the alkalies. These familiar fluxes would tend to rise 

 toward the top of the magma chamber. A given amount of the acid 

 would transfer a larger amount of either soda or potash. With falling 

 temperature and at the pressures appropriate to the upper part of the 

 chamber, the carbon dioxide would be slowly replaced by silica and ex- 

 pelled through the roof of the chamber. On the other hand, the reaction 

 may be much more complicated than that suggested, and, in fact, it is 

 likely that the alkalies often move upward in the form of alumino- 

 silicate. The problem illustrates the darkness yet shrouding the opera- 

 tions of the "agents miner alisateurs.^^ 



That there is an actual transfer of one or both alkalies is shown in 

 such examples as basanite, leucite basalt, and nephelinite.* The alkaline 

 contents of these rocks can not be explained as features of magmas which 

 are residual after the settling out of femic and cafemic constituents from 

 normal basalt. 



Without speculating further as to the nature of the physico-chemical 

 changes, we may note that both acid and basic elements of dissolved car- 

 bonates cooperate in concentrating alkalies in the upper part of a column 

 of basalt. A similar argument and conclusion may be derived for other 

 subalkaline magmas. 



Additional Evidences favoring the Hypothesis 



Besides the facts of field association and the high probability of appro- 

 priate physico-chemical reactions, there are a number of important con- 

 siderations favoring the general thesis of this paper. 



Many special pyrogenetic minerals, more or less peculiar to the alka- 

 line rocks, herewith find that explanation which is often so difficult on 

 the supposition that these rocks have crystallized from primary magma. 

 The list includes: 



First. Minerals with carbon dioxide as an essential component : calcite 

 and cancrinite of nephelite syenite, etcetera. 



Second. Minerals showing desilication of normal subalkaline magmas : 

 nephelite, leucite, muscovite (Pin part ?), sodalite, haiiynite, analcite, 

 corundum, spinel. 



Third. Minerals formed through the presence of excess lime : melilite, 

 scapolite, wollastonite, garnet, titanite (in part), calcite, anorthoclase 

 (in part ?), perovskite. 



* See table of average compositions of these and ottier types in Proceedings American 

 Academy of Arts and Sciences, vol. 45, 1910. pp. 228-229. 



