GEOLOGY OF THE NORTHERN ADIRONDACK REGION 307 



treme. As has been seen, that consists (1) in a change in com- 

 position due to increasing amount of pyroxenes and ilmenite with 

 corresponding diminution of feldspar and the appearance of gar- 

 net in quantity; (2) in a change in texture, the rock becoming 

 less coarsely crystalline and the large feldspars diminishing in 

 frequency and size,, with increase in the amount of granulated 

 materia] ; and (3) in the rock becoming more prominently foliated 

 with concentration of the dark minerals along the cleavage planes. 

 In this further change the dark minerals come to form 50# or 

 more of the rock; the large feldspars become constantly smaller 

 and less frequent up to complete disappearance; and the rock 

 becomes eventually a finely granular, well foliated, dark gneiss. 

 The change from the anorthosite into this rock is gradual, and 

 the relationship unmistakable ; yet to an observer first coming on 

 the rock from without the anorthosite area, such a relationship 

 would seem most improbable. This gneiss is just as clearly 

 a border differentiation product of the anorthosite gabbro as that 

 is of the anorthosite, a differentiation produced in the molten mass 

 after it had reached its present resting place and while cooling. 

 It is however impossible to say why this further differentiation 

 has taken place only on this one side of the mass instead of rather 

 uniformly about the whole, as is the case with the anorthosite 

 gabbro. 



Surrounding rocks. The Franklin county anorthosite is bor- 

 dered by all sorts of rocks, both Grenville and doubtful gneisses 

 and later igneous rocks. That it is younger than the Grenville 

 and some of the doubtful gneisses is definitely established by 

 (1) the fact that masses of varying size are found inclosed in 

 the anorthosite; (2) by the fact that the few contacts exposed 

 show the anorthosite cutting them and sending tongues into them ; 

 and (3) by the fact that, where no contacts are exposed, the anor- 

 thosite can be shown to cut out the other rocks along their strike. 

 The later date of certain igneous rocks, shortly to be described, 

 is mainly deduced from the finding of dikes of what are thought 

 to be identical rocks which cut the anorthosite. So far as the 

 actual dikes are concerned, there can be no question of their later 



