442 Williams — Gabbros and Diorites of the Cortlandt Series. 



Specimen Mt. 7 of Prof. Dana's collection is interesting as 

 illustrating the alterations which one of the coarser hornblend- 

 ites from Montrose Point has undergone. The change of the 

 brown hornblende is to serpentine and talc. 



Hornblendite composed entirely of green hornblende is rare 

 within the Cortlandt Area. It does, however, occur among the 

 dykes intersecting the limestone at Yerplanck Point, as shown 

 in section YK.l. of Prof. Dana's collection. 



3. Green-hornblende-Diorite. — Typical diorites of this class 

 are not common in the Cortlandt Area. Those observed occur 

 in narrow dykes on Montrose or Yerplanck Points. These 

 diorites which are wholly free from biotite always contain a 

 hornblende, which, though it may properly be called green, has 

 .nevertheless a decidedly brownish tinge. On the whole the 

 relationship of these rocks with the brown hornblende diorites 

 is much closer than it is to those of the following class. 



By far the most typical development of the green-hornblende 

 diorites belonging to the " Cortlandt Series," occurs along the 

 edge of the steep rock wall which extends westward from 

 Cruger's Station, toward Montrose Point. This abrupt ascent 

 marks the contact between the massive rocks and the softer, 

 though much metamorphosed schists. These diorites, however, 

 always carry a large amount of biotite and therefore are more 

 properly classed as 



4. Mica-hornblende- Diorites. — The association between this 

 type and the pure mica-diorite (class Y) is extremely intimate 

 and there is everywhere observable a tendency toward the de- 

 velopment of the latter rock by the total replacement of the 

 hornblende by the biotite. 



The most prominent microscopical peculiarity of these green 

 diorites is their sudden and extreme alterations of grain ; very 

 coarse and fine varieties occurring side by side in the same ex- 

 posure, in a manner unequalled in any other part of the entire 

 Cortlandt Area. The best locality to observe this structure is 

 just above the brick-sheds near Cruger's- Station, at a point 

 marked (i jp " on Professor Dana's map.* 



The constant mineral constituents of these diorites are a 

 finely striated plagioclase, green compact hornblende, biotite, 

 magnetite, epidote and apatite. Less abundant are an unstri- 

 ated feldspar (orthoclase) and quartz. Garnet is a frequent 

 endo-metamorphic product near the contact of the diorite with 

 the schists. The hornblende differs only in its color from the 

 compact brown hornblende already described in other mem- 

 bers of the "Cortlandt Series." It occurs in irregular indi- 

 viduals which are filled with magnetite inclusions. The color 

 is a deep green, often inclining to bluish-green, and the pleo- 

 chroism is very intense. 



* Loc. cit. 



