OBJECTIONS TO HYPOTHESIS OF ORIGIN 113 



cases where limestone is extremely Yolnminous as compared with the acid 

 rocks invaded, a granitic batholith may be locally desilicated and differ- 

 entiated so as to produce nephelite syenites. Illustrations are seen in the 

 great batholiths of eastern Ontario, as described l)y Adams, Barlow, 

 Coleman, and others.^ Many other bodies, smaller than batholiths, have 

 no alkaline differentiates. Most of them were clearly too small to have 

 assimilated any notable quantity of foreign rock, even of limestone. 

 Some bodies have failed to assimilate because of their low temperatures. 

 Where thermal and other conditions are necessarily so variable, each case 

 has evidently to he studied hy itself before its bearing on our general 

 hypothesis is visible. 



Alkaline Eocks not associated with calcareous Sediiments 



It would not be surprising if in future time it is demonstrated that 

 some of the so-called alkaline rocks were evolved under conditions essen- 

 tially different from those controlling the formation of such t3^pes as 

 phonolite, nephelite syenite, theralite, etcetera. For example, many alka- 

 line granites which cut granodiorite masses are most probably salic differ- 

 entiates of the granodioritic magmas. In many of these cases the dif- 

 ferentiation seems to be a result of the spontaneous splitting of the 

 magmas rather than an effect of their absorption of foreign material. 

 The splitting may have been assisted by the concentration of gases in 

 part of the batholithic chamber, but those gases may he juvenile and not 

 ^'resurgent."^ 



An analogous cause for the differentiation of phonolite has been sug- 

 gested in the case of the Cripple Creek district. The alkaline masses of 

 that region occur on the top of the great dome which, in Tertiary time, 

 was upwarped to form the Front Eange of Colorado. The development 

 of such a dome must mean an extensive inflow of the plastic material on 

 which the deformed granite rests. The disturbance might readily be con- 

 ceived to lead to the concentration of juvenile gases, including carbon 

 dioxide, beneath the domed granitic shell. A late Tertiary abyssal injec- 

 tion of basaltic magma would naturally carry much of this accumulated 

 gas; the differentiation of this magma might give the latite-phonolite, 



8 F. D. Adams and A. E. Barlow : Transactions of the Royal Society of Canada, third 

 series, vol. 2, section 4, 1909, p. 8. A. P. Coleman : Journal of Geology, vol. 7, 1899, p. 

 437. 



» The writer uses "resurgent" as a technical term to designate those volatile sub- 

 stances which, as constituents of sedimentary rocks assimilated by magma, have been 

 given off from the secondary, syntectic magma. Juvenile gases, on the other hand, are 

 those emanating for the first time from the earth's interior. See American Journal of 

 Science, vol. 26, 1908, p. 48. 



VIII— Bull. Geol. Soc. Am.. Vol. 21. 1900 



