Williams — Gabbros and Diorites of the Cortlandt Series. 439 



with Cortlandt township — is mainly composed of norite, the 

 many varieties of which were described in my last paper. In 

 the southeastern and southwestern corners of the township, as 

 well as on Stony Point on the opposite side of the Hudson 

 River, olivine-norites and peridotites are found, while at other 

 localities, mostly in the southwestern portion of the area, 

 still different but closely allied types of massive rocks occur. 

 These, which form the subject of the present communication, 

 are : — 



Class III, Gabbro, 



Class IY, Diorite, 



Class Y, Mica-Diorite. 



These rocks are everywhere connected so closely by inter- 

 mediate forms that they may, to a certain extent, be regarded 

 as facies of the norite. Indeed, even in the types most widely 

 removed from the prevailing rock hypersthene is very liable 

 to recur. There are enough general resemblances and con- 

 necting links to join all the rocks of this series into a geologi- 

 cal unit ; and at the same time there are differences sufficient 

 to show that many types were successively produced from the 

 same igneous focus. 



Class III. Gabbro (von Buch.) 



1. Gabbro proper. — This rock is to be found at only a few 

 isolated localities, of which the most representative is " Mun- 

 ger's Corners," a short distance west of Montrose Station on the 

 N. Y. C. & H. R. R. Prof. Dana has designated this place as 

 " g " on his map, and describes the occurrence as " a grayish 

 white augitic rock."* It is represented by several slides in 

 both Prof. Dana's and the Johns Hopkins University collec- 

 tion. (No. 42 and K, Mt. 13 (D)). 



Under the microscope this rock appears as an aggregate of 

 allotriomorphous plagioclase and augite grains. The latter 

 mineral is of a reddish or grayish color, both often appear- 

 ing in the same crystal individual. It is without pleochroism 

 and frequently shows a pronounced orthopinacoidal parting. 

 The substance of the augite or diallage is for the most part un- 

 altered, although a little green uralite is occasionally devel- 

 oped. Accessory constituents in this rock are biotite, apatite, 

 ilmenite and sphene. The last named mineral is quite abun- 

 dantly represented in all sections and appears to have resulted 

 from the alteration of the titanic iron. 



The gabbro shows evidence of great dynamic action. The 

 twinning lamellae of the plagioclase are much curved and both 



* This Journal, III, xx, p. 195, and p. 211, Sept. 1880. 



