Williams — Gabbros and Diorites of the Cortlandt Series. ^M. 



The two types of diorite produced by the presence of brown 

 or of green hornblende are quite distinct both in their occur- 

 rence and relationships. The former is always associated with 

 pyroxene rocks and tends to pass gradually into norite, gab- 

 bro, or pyroxenite ; by a total loss of feldspar these diorites 

 may also develop into massive hornblendites. The diorites 

 composed of green hornblende, on the other hand, show their 

 closest relationship to the mica-bearing rocks. 



The grain of these diorites varies extremely, from apha- 

 nitic varieties to such as have hornblende individuals over six 

 inches in length. 



1. Brown-hornblende-Diorite. — This type is best developed 

 in the wonderfully complicated net-work of massive rocks ex- 

 posed on the river bank along the northern portion of Mon- 

 trose Point. The brown diorite is most intimately associated 

 with norite, and grades, on the one hand into this, and on the 

 other into a massive brown hornblendite. The other constitu- 

 ents are triclinic feldspar (presumably the same andesine as 

 occurs in the norites),* apatite and magnetite. Accessory hy- 

 persthene is common by which the diorite shows its tendency 

 to grade into the norite. 



The brown diorites extend, with exactly the same associa- 

 tions, eastward from Montrose Point nearly as far as Mon- 

 trose Station, as is shown by a large number of sections in 

 both the University and in Prof. Dana's collection. They 

 were, however, not encountered in other parts of the Cort- 

 landt Area. 



2. Hornblendite. — Both coarse- and fine-grained aggregates 

 of compact brown hornblende occur abundantly along the 

 northern portion of Montrose Point. These rocks have a glis- 

 tening black color and are most intimately associated with the 

 norites, hyperites, diorites, and pyroxenites which also occur 

 there. No more complicated interpenetration of eruptive 

 rock-types could possibly be imagined than is displayed at this 

 locality — every rock includes and forms dykes in every other ; 

 and at the same time every type passes by gradual changes in 

 its mineralogical composition into every other one ! 



The striking examples of the passage by paramorphism of 

 pyroxene into compact brown hornblende described some time 

 since by the writer, f occur in rocks from Montrose Point 

 intermediate between pyroxenite and hornblendite. The 

 origin of the brown hornblende from both the diallage and 

 the hypersthene is so apparent as to suggest the derivation 

 of all the hornblendites by paramorphism from preexistent 

 pyroxenites. 



* This Journal, III, xxxiii, p. 140, Feb., 1886. 



f This Journal, III, xxviii, p. 261, et seq., Oct., 1884. 



