GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 39 



Interpretation. The possibility suggested itself that some of 

 these rocks might be members of the basal gneiss series. The 

 quartzite, however, is almost certainly Paleozoic in age. The black 

 hornblende rock has the characters of a basic eruptive. The green 

 .serpentine variety gives little idea of its original character, but it is 

 apparently not an altered pyroxenic rock. 



The southwestward continuation of the reversed fault along the 

 northwestern slope of the eastern gneiss mass would apparently 

 intersect the fault on the east of Shenandoah mountain in this 

 neighborhood. The latter scarp is only a short distance west of the 

 quartzite. This intersection would have been a most favorable point 

 for an igneous intrusion. Some of the basal Paleozoics were evi- 

 dently caught at this intersection and intruded by the hornblende 

 rock. The quartzite offered little for the eruptive to act upon. The 

 bastite rock very probably represents an impure ferruginous dolo- 

 mite. From what is known of bastite, it is commonly, at least, the 

 alteration product of orthorhombic pyroxene; but the present rock 

 gives no indication of the former presence of any antecedent min- 

 eral. There seems to be no grave objection to the inference that 

 the passage was direct.^ 



This is the only occurrence within the quadrangle that permits 

 the interpretation that an eruptive has penetrated and altered the 

 overlying Paleozoics. 



THE BASAL QUARTZITE (POUGHOUAG) 



Distribution and general structural features. This formation, 

 which has frequently been mentioned in connection with the 

 gneisses, in this quadrangle occurs only in proximity to the Pre- 

 cambric rocks. 



In the town of Matteawan the quartzite forms a small inlier as 

 described above, in connection with the first small inlier of gneiss 

 south of the Glenham belt (see page 28). Outcrops were also seen 

 just north of Rowland avenue in the open field at the foot of the 

 Mount Beacon incline. The only other outcrops which have been 

 noted in this vicinity occur farther north along the base of Bald 

 hill on the Maddock estate.^ About 300 yards south of the house 

 and well up in the woods, about 200 or 300 feet east of the private 



1 Professor B. K. Emerson assisted the writer in the identification of 

 the mineral bastite. 



^ The presence of the quartzite at this point was discovered by a com- 

 panion, Mr W. R. Clarke. 



