ABSENCE OF GRENVILLE AND SYENITE-GRANITE 453 



generally rather highly fluid syenite-granite magma. That the syenite- 

 granite magma was rather highly fluid is proved by its great power to 

 cross-cut, intimately penetrate, break up, and tilt the Grenville strata. 

 Only exceptionally did local portions of this magma invade the Grenville 

 strata in true laccolithic fashion. 



Both the anorthosite and the syenite-granite, I believe, intruded a very 

 thick mass of essentially undisturbed Grenville strata, largely or alto- 

 gether free from orthogneiss. The southwestern half of the anorthosite 

 body, which is so free from masses of Grenville and syenite-granite, I 

 believe represents the greatest bulk of the anorthosite where the lacco- 

 lithic magma was thickest and reached the highest level. The northeast- 

 ern half of the anorthosite, as now exposed, I regard as the portion where 

 the magma spread out as a relatively much thinner layer w^hose surface 

 was at a notably lower level than that of the thicker portion to the south- 

 west (see figure 3). Because of the greater uplift of the southwestern 

 portion the Grenville cover has been almost, if not completely, removed 

 by erosion. But many areas of the Grenville roof remain over the thinner 

 northeastern part where the uplift was much less. Thus we have a simple 

 explanation of the absence of the Grenville from so much of the anor- 

 thosite area. 



After the solidification of the great body of anorthosite, the syenite- 

 granite magma was batholithically intruded in a rather highly fluid state 

 and tended to avoid penetration of the anorthosite which was much more 

 massive, homogeneous, and resistant than the great mass of surrounding 

 practically undisturbed Grenville strata. This satisfactorily explains not 

 only why syenite-granite masses are scarcer within the anorthosite area 

 than in the Adirondack region in general, but also v^hy syenite-gran it a 

 masses are almost, or entirely, absent from the southwestern half. I do 

 not believe it necessary to assume that the feeding channel of the lacco- 

 lith was as small as generally represented in diagrams of laccoliths. May 

 not there have been a wide feeding channel extending northwest by south- 

 east under the main body of the southwestern half of the anorthosite? 

 On this view, the thickest portion of the laccolith developed directly over 

 the wide feeding channel which extended far down, with the result that 

 this portion of the anorthosite intrusive body was very resistant to intru- 

 sion by the syenite-granite magma. The northeastern portion of the 

 anorthosite, because notably thinner, was penetrated by considerable 

 masses of the syenite-granite magma, as in the Lake Placid and Ausable 

 quadrangles. Here, again, v^e have a simple explanation of the field facts. 



If we do not accept the hypothesis of stratiform arrangement of the 

 syenite-granite and anorthosite, we are therefore not forced, as Bowen 



