NORMAL SEQUENCES 631 



studied series. If any such series can be studied it will be a valuable 

 addition to the list of differentiated regions. 28 



Syntexis 



Since the normal sequence of rocks from gabbro to granite can be so 

 well explained by crystallization from basaltic magma, the argument for 

 syntexis has recently been based more largely on the occurrence of alka- 

 line rocks, for which crystal settling seems a less satisfactory explanation. 

 Daly and Foye argue for the effect of basic sediments as controlling this 

 change from the main part of the normal series to an alkaline type. 29 



The nephelite syenites scatter widely in the diagrams, but it may be 

 remarked that practically every series including nephelite ; syenite passes 

 very close to the average nephelite syenite. Some series from nephelite 

 syenite to subalkalic rocks run to gabbro and some to granite. Is it pos- 

 sible that the assimilation of sediments is so well regulated by nature as 

 to give the same product, nephelite syenite, no matter what the composi- 

 tion of magma to start with? In case of granite, the assimilated ma- 

 terial would desilicate the magma and increase the alkalies a little with- 

 out leaving much lime in the desilicated magma. In the case of gabbro, 

 the alkali content would need to be enormously increased and ferro- 

 magnesian minerals eliminated. The uniformity of product is hardly 

 to be expected from assimilation without a more definite control by dif- 

 fer entation. And if differentation is the chief control, it seems sufficient 

 to explain the results without any important relation to assimilation. 



Primary Magma 



the problem 



As recently as 1911, Loewinson-Lessing presented somewhat elaborate 

 arguments for at least two primary earth magmas, basaltic and granitic. 30 

 This was in continuation of similar theories which had been argued for 



28 Probably tbe best approach to such an example at present is the Haliburton-Ban- 

 croft area, where the nephelite rock is related to both essexite and granite ; but even 

 there no connection has been shown between granite and a basaltic magma. In central 

 Wisconsin, where series have been found including gabbro, granite, and alkaline types, 

 the geologic connection is more remote and gradations have not been reported. With 

 the present data, shown in the D and G lines of figure 4, it would be hard to predict 

 where the lines should be connected. 



29 R. A. Daly : Igneous rocks and their origin, p. 414. 



W. G. Foye : Nephelite syenites of Haliburton County, Ontario. Am. Jour. Sci., vol. 

 40, 1915, pp. 428-436. 



30 F. Loewinson-Lessing : Origin of igneous rocks. Geol. Mag., vol. 8, 1911, pp. 248- 

 297. 



