THE PROBLEM OF THE ANORTHOSITES 223 



sary to use a color to represent a mixture of Grenville and syenite 

 that defies separate mapping. Now the manner of occurrence of 

 the Grenville when found in considerable areas is commonly as a 

 roof lapping over the syenite and showing only comparatively 

 moderate dips. One sees this in typical form on the shores of 

 Lake Champlain immediately north of Port Henry, and Miller has 

 recently described this relation in widely scattered Adirondack 

 localities, the large-scale example in the Blue Mountain quadrangle 

 being of special interest. 1 Syenite and Grenville in this relation 

 are almost constant companions. 



The anorthosite areas, on the other hand, are very different. It 

 could be said with little exaggeration that on passing the borders of 

 the anorthosite core one encounters only anorthosite. It is true 

 that inclusions of Grenville have been found, enough to prove the 

 intrusive nature of the anorthosite, but these appear to be small 

 completely inclosed blocks and do not suggest actual roof remnants. 

 In spite of their occasional occurrence the contrast between the 

 syenite and anorthosite areas is very striking. One has but to 

 glance at the maps of such areas as the Paradox Lake and Long Lake 

 quadrangles to be convinced of it. Not only is the anorthosite 

 unbroken by areas of Grenville, especially away from the margins, 

 but it is likewise practically free from protrusions of the syenite, 

 although the syenite is, as we have seen, in part at least, a later 

 rock. If one pictures the syenite and the anorthosite as conven- 

 tional batholiths, some difficulty is experienced in accounting for 

 the foregoing facts. It is necessary to imagine an early intrusion of 

 a huge plug of anorthosite followed by an intrusion of syenite which 

 took the form of a hollow cylinder circumscribing it and invading 

 it only peripherally. All of this must take place without throw- 

 ing the Grenville series into appressed folds, indeed, without very 

 significant folding of any kind. It is then necessary to imagine 

 that erosion removed every vestige of a roof from the small interior 

 anorthosite area, and left great stretches of it throughout the broad 

 syenite-granite belt that surrounds it. 



All of this is perhaps possible, but at the same time seems highly 

 improbable. On the other hand, if one pictures the Adirondack 



1 William J. Miller, Jour. Geol., XXIV (1916), 591. 



