440 Williams — Gabhros and D writes of the Cortlandt Series. 



the feldspar and the augite are often peripherally granulated 

 by crushing and rubbing. 



Another rock (No. 44) occurring at Centerville on the south 

 side of Prof. Dana's limestone 4,* is in all respects identical 

 with the gabbro at Munger's Corners. 



The eruptive dykes which occur in such intimate association 

 with the limestone at the southern end of Yerplanck Point, 

 are in part gabbros ; in part mica- or hornblende diorites. No. 

 Ill, from one of the narrowest of these dikes, is quite like 

 the gabbros last described, except that it is finer grained. Its 

 augite also is more extensively changed to uralite. The thin 

 section of this specimen includes some of the limestone in con- 

 tact with the eruptive rock. This is altered by the meta- 

 m orphic action into a granular aggregate of pale green py- 

 roxene together with some pale hornblende and pleonaste. 



2. Mica-Gabbro. — The presence of accessory biotite in the 

 gabbros has been mentioned above ; in some cases this min- 

 eral becomes so largely developed as to equal or even exceed 

 the amount of augite present. Thus No. 109 and VK 5, of 

 Prof. Dana's collection, both from dikes at Yerplanck Point, 

 differ only from the normal gabbro of this locality in the in- 

 creased amount of biotite present. No. 45 also is only a bio- 

 tite modification of the Centerville gabbro above mentioned. 



The most interesting point in regard to the gabbros of the 

 Cortlandt Area is that they always (so far as observations yet 

 extend), occur immediately beside limestone. They seem to 

 represent a local modification of the norite produced by an 

 increase of lime, for this, as is well known, would change the 

 orthorhombic magnesian hypersthene to a monoclinic pyroxene. 



Class IV. Dioeite. (Haily.) 



The hornblende, which imparts the essential character to this 

 class of rocks, is compact and homogeneous in structure, pos- 

 sessing every appearance of a primary constituent. It occurs 

 in allotriomorphous individuals which vary in size according to 

 the coarseness of the rock-grain. In the main this hornblende 

 is identical with that already described at length from the 

 hornbleude-periclotite of Stony Point, f In some instances 

 this hornblende contains the same delicate inclusions, while in 

 others these are totally wanting. Its pleochroism is always 

 strong, and its color either a deep chestnut brown or a bright 

 green. More rarely it shows by transmitted light a color in- 

 termediate between these two. 



* Vid. the map in Prof. Dana's article. This Journal, III, xx, p. 195, Sept., 

 1880. 

 \ This Journal, III, xxxi, p. 31, Jan. 1886. 



