CHILLED GABBROID BORDER FACES 417 



THE BORDER FACIE 8 BOTH AN OUTER AND AN UPPER LIMIT OF THE 



ANORTHOSITE 



Cushing^^ maintains that the gabbroid border facies in its present posi- 

 tion marks the outer limit of the great body of Adirondack anorthosite, 

 or, in other wards, "that the chilled border determines for ns the original 

 size of the mass at the depth represented by the present erosion surface," 

 and that therefore the "anorthosite mass can not be regarded as spreading 

 out underneath the outlying syenite masses and extending throughout the 

 (Adirondack) region." This is a very important point and, since Cush- 

 ing's argument in support of it seems to me to be conclusive, it is unnec- 

 essary to repeat it here. Though I agree with Cushing's main argument, 

 there is one slight modification, namely, that the borders of the anortho- 

 site body have in some districts, like the Schroon Lake and Lake Placid 

 quadrangles, been so cut out or cut to pieces by the syenite-granite intru- 

 sion that the full original extent of the anorthosite is not now show^n. 

 This is a matter of some miles in certain districts. 



There is strong evidence that the chilled gabbroid border facies also 

 developed as an upper limit which formerly existed as a cover resting 

 directly on the whole great mass of anorthosite rather than merely as an 

 outer limit, as Gushing suggests. Thus, as already pointed out, the 

 Whiteface anorthosite of the Lake Placid quadrangle does not exist 

 merely as a definite fringe around the outer margin of the Marcy anor- 

 thosite. I have there found typical Whiteface anorthosite fully 14 or 15 

 miles within the present border of the anorthosite area, and inclusions in 

 the syenite-granite series outside the general anorthosite area show that 

 the Whiteface anorthosite formerly extended at least a few miles farther 

 out than the present boundary. One area of Marcy anorthosite, 12 miles 

 long within the Lake Placid quadrangle and extending an unknown dis- 

 tance into the Ausable quadrangle, is flanked on either side by Whiteface 

 anorthosite. It is hard to resist the suggestion that Whiteface anortho- 

 site formerly covered this whole mass of Marcy anorthosite. There is 

 thus a distinct difficulty in the way of considering this Whiteface anor- 

 thosite merely an outer border facies. But if we do so regard this border 

 facies, we are forced to conclude that it is exceedingly thick — that is to 

 say, fully 10 or 15 miles — the width of the area containing Whiteface 

 anorthosite representing practically the thickness of the border phase. 

 This is scarcely possible. If, however, we consider the Whiteface anor- 

 thosite of the quadrangle to mark an upper limit of the great anorthosite 

 body, but now partially removed by erosion and partly cut into by the 



27 H. p. Gushing: Jour. Geol., vol. 25, 1917, p. 506. 



