108 K. A. DALY ORIGIN OF THE ALKALINE ROCKS 



For 107 of the 155 districts tabulated the rule connecting alkaline 

 eruptives and carbonate rocks is quite clear. In many ot the remaining 

 cases, including most of the oceanic islands, full information as to the 

 nature of the formations invaded by the magmas could not be found in 

 the literature or in geological maps. The phonolitic intrusives and lavas 

 of the Cripple Creek district, Colorado, cut pre-Cambrian granites, and 

 there is little probability that their magma could have made contact with 

 any limestone overlying those granites. The alkaline rocks of Eed Hill, 

 Few Hampshire, cut orthogneiss which seems to be similarly barren of 

 carbonate beds, but it is possible that limestones occurred in the roof of 

 the Eed Hill stock; or it is possible that both the Eed Hill and Cripple 

 Creek granites traversed by the alkaline magmas had incorporated (stoped 

 down) large masses of limestone during the earlier, batholithic intrusion, 

 and that these limestones were cut by the foyaitic magmas. That this 

 speculation is not entirely fantastic is shown by the fact that the deeper 

 mine workings of the Cripple Creek district are seriously affected by 

 abundant emanations of carbonic acid gas ; that gas comes from below or 

 from the volcanic rocks in which the levels are run and the shafts sunk; 

 or, finally, it is possible that the Cripple Creek, Eed Hill, and some other 

 alkaline magmas have nowhere made contact with carbonate rocks, though 

 those magmas were for some special reason unusually rich in juvenile 

 carbon dioxide. 



These exceptional cases suggest the hypothesis that it is the presence of 

 carbon dioxide rather than of dissolved carbonate in subalkaline magma, 

 which is the principal condition for the differentiation of fractions high 

 in alkalies. The following discussion will, however, refer chiefly to the 

 effect of the assimilation of limestone and dolomite, with their combined 

 carbon dioxide, by subalkaline magma. 



Effects of the Solution of Carbonates in subalkaline Magma 



Needless to say, the experimental data are yet insufficient for a com- 

 plete physico-chemical analysis of this subject. What follows is but a 

 series of the more manifest suggestions or inferences derived from general 

 studies of silicate melts and of the solid rocks. Incomplete as the deduc- 

 tions may be, they all point toward the one conclusion — ^that subalkaline 

 magma must suffer such chemical modification by the assimilation of car- 

 bonates as will lead to fractions which are either abnormally rich in the 

 alkalies or are so far desilicated as to yield, by crystallization, feldspath- 

 oids instead of feldspars. The various points will be very briefly stated, 

 for it is the writer's purpose to draw to the main hypothesis the attention 



