1J2 R. A. i)ALY ORIGIN OF THE ALKALINE ROCK^ 



SO refractory a rock as gneiss or granite. The temperature of the active 

 Kilauean vent is at least 1200° centigrade. If the fusion or solution of 

 gneiss takes place at any such temperature, we must believe that fluxing 

 carbonate can enter into solution with the molten basalts of nature. It 

 appears that any subalkaline magma would be similarly fluxed. A few 

 rock types such as melilite basalt probably represent syntectics which 

 have differentiated only slightly, but, in general, differentiation will be 

 specially active in limestone syntectics. Typical hybrid rocks are, there- 

 fore, not to be expected in most districts. On the contrary, differen- 

 tiates poor in lime are the result of the absorption of limestone by sub- 

 alkaline magmas. Such a differentiate is found near the top of the 

 magma chambers. The known structural relations of nephelite syenites 

 and of phonolites are appropriate to this position of the extreme polar 

 differentiate in intrusive mass or lava column. If magmatic stoping of 

 limestone takes place, the ensuing assimilation is abyssal, and the actual 

 syntectic solution or rock may never be accessible to direct observation. 

 Finally, it must be remembered that most stock and batholith contacts 

 have been established after molar-contact assimilation has become almost 

 impossible in the already much cooled magma. The question of mag- 

 matic assimilation can never be so easily settled as by a mere inspection 

 of main contacts or of xenolithic inclusions. To deny the efficiency of 

 magmatic assimilation on such evidence is scarcely a scientific judgment. 



A conceivable objection to the hypothesis is that limestones and other 

 calcareous sediments are very generally distributed over the earth's sur- 

 face, and accordingly the observed association of alkaline rocks and the 

 sediments is accidental and without significance.^ A final reply to this 

 objection would involve a full discussion of the field relations of the sub- 

 alkaline rocks. This colossal task need not be undertaken, for a partial 

 review of the subalkaline rock occurrences must make it clear to every 

 one that very many, perhaps most, of these bodies have not contacted 

 with limestone during eruption. 



Again, the hypothesis must take account of a considerable number of 

 cases where subalkaline rocks now make contact with masses of calca- 

 reous sediment and are not accompanied by rock types which are evi- 

 dently alkaline by our definition of the term. This is true of many 

 batholiths which have absorbed much silicious rock as well as limestone. 

 The net result has been to give an acid syntectic and then quartzose or 

 feldspathic differentiates at the batholithic roofs. On the other hand, 

 the absorbed sediments are often so great in amount as to give syenite 

 instead of granite. If the limestone is relatively abundant, the syenite 

 may be of nordmarkitic, pulaskitic, or other alkaline type. In the rare 



