456 W. J. MILLER ADIRONDACK ANORTHOSITE 



Daly/^ after considering a number of the better known anorthosite 

 bodies of the world, concludes that all of them, including the Adirondack 

 mass, are best to be regarded as laccoliths. 



In this paper I have presented many field facts which best harmonize 

 with the conception of a laccolithic structure of the Adirondack anortho- 

 site. As already shown, it is not at all necessary to assume, as does Bowen, 

 that both the syenite-granite series and the anorthosite were batholithic 

 because they are regarded as distinctly separate intrusions. I believe the 

 anorthosite body (not necessarily anorthosite as such) was intruded essen- 

 tially laccolithically, while the syenite-granite series was intruded essen- 

 tially batholithically. 



Positive proof for the laccolithic structure of the Adirondack anortho- 

 site can not be won from a study of its relation to the intruded Grenville 

 strata. In the first place, only a very few (usually small) areas of Gren- 

 ville are known to lie against the borders of the anorthosite, having been 

 cut out so extensively by the syenite-granite series, and these contacts are 

 almost all concealed under Pleistocene deposits. In the second place, 

 such Grenville strata were more or less disturbed again by the later sye- 

 nite-granite intrusion. 



My reason for regarding the anorthosite as essentially a laccolithic 

 body is that many of the important field facts support this conception, 

 while none are opposed to it. Since I have already presented and dis- 

 cussed these facts, I shall not repeat the details here. Among the many 

 facts in harmony with the laccolithic conception are the following: The 

 chilled border facies, which developed as an upper as well as an outer 

 border, resting directly on and against the Marcy anorthosite; failure to 

 find masses (even small inclusions) of Grenville country rock farther 

 within the anorthosite than just below the inner margin of the chilled 

 border, thus indicating the power of the anorthosite to have lifted rather 

 than to have extensively cross-cut or engulfed Grenville; failure of both 

 the syenite-granite series and the gabbro stocks to penetrate the main 

 body of the southwestern half of the anorthosite, and moderate penetra- 

 tion of the northeastern half by the rocks just named, thus very strongly 

 suggesting a laccolith very thick toward the southwest and relatively thin 

 tov^ard the northeast. 



PROBABLE ORIGIN OF THE ANORTHOSITE BY SETTLING OF FEMIC 

 CONSTITUENTS 



It might be possible to conceive the anorthosite to have been intruded 

 in the form of a laccolithic magma whose composition was practically the 



« R. A. Daly : Igneous Rocks and Their Origin. 1 ftl 4, pp. 3'^8-o3o. 



