ABSENCE OF GRENVILLE AND SYENITE-GRANITE 455 



Elizabetlitown, Paradox Lake, Schroon Lake, and Long Lake maps show 

 no gabbro well within the margin of the anorthosite, except a very small 

 mass 2 or 3 miles within the border in the Long Lake quadrangle. So 

 far as I know, none occurs in the southern half of the Mount Marcy 

 quadrangle. On the southern side of the anorthosite area gabbro stocks 

 are present in unusual abundance right up to the very border of the anor- 

 thosite, beyond which none are found. In the Schroon Lake and Eliza- 

 bethtown quadrangles a number of gabbro stocks from 2 to 4 miles long 

 lie right along the border. In the northeastern portion of the anorthosite 

 area, however, small gabbro stocks occur in moderate numbers, as, for 

 example, in the Lake Placid quadrangle. 



It is, then, very clear that this later gabbro shows the same sort of dis- 

 tribution with reference to the anorthosite as does the syenite-granite, and 

 I believe the same explanation (see above) applies to both. Evidently 

 the gabbro intrusions were unable to penetrate the thick, very resistant 

 southwestern half of the anorthosite laccolith. Since the gabbro stocks 

 are distinctly intrusive and later than both the anorthosite and the sye- 

 nite-granite, their absence can not be explained as due to removal by 

 erosion from a large part of the anorthosite area. How would Bowen 

 explain the fact that this later gabbro shows exactly the same remarkable 

 distribution with reference to the anorthosite as does the syenite-granite 

 series? Is the absence of this gabbro from so much of the anorthosite 

 area any less remarkable and explainable in a different way than the 

 absence of the syenite-granite ? 



Origin of Anorthosite by Differentiation in a Laccolith of 



Gabbroid Magma 



laccoljthig structure of the anorthosite 



According to Bowen, the anorthosite and the syenite-granite series are 

 both essential parts of a single vast laccolithic body whose diameter is at 

 least as great as that of the whole Adirondack region. To say the least, 

 this would be a tremendous laccolith. The main thesis of my paper has 

 been to show that this hypothesis is untenable in the face of many impor- 

 tant field facts. 



In his recent paper. Gushing states that he does not object ^% the lac- 

 colithic conception, but to the conception of a single laccolith occupying 

 the entire region.^' He argues that the anorthosite is a separate intrusive 

 body distinctly older than the syenite-granite series, but he does not pre- 

 sent evidence for the laccolithic structure of -the Adirondack anorthosite. 



