FETTKE, MANHATTAN SCHJST OF NEW YORK 233 



apparently derived from a pyro.xciie, while the aiitigorite represents 

 altered olivine, as it shows the typical mesh structure and occasionally 

 contains cores of unaltered olivine. Some of the serpentine from the 

 same locality has a slightly banded structure. A thin section of this 

 phase shows a distinctly foliated structure under the microscope and 

 consists largely of tremolite oriented parallel to the foliation, with anti- 

 gorite filling in the space between its prisms as well as the little crevices 

 and cracks throughout the section. The tremolite is perfectly fresh and 

 shows no alteration to serpentine. Some calcite is also present in the 

 section. The accessory minerals are fairly abundant pleonaste and 

 magnetite. 



As has already been pointed out, these serpentine masses undoubtedly 

 represent altered basic intrusive rocks rich in olivine. From their mas- 

 sive structure, it would appear that they entered either toward the close 

 of the period of folding or after it had come to an end. They are, there- 

 fore, probably closely related to the Cortlandt series, the Croton Falls 

 hornblendite and the Bedford diorite which have already been discussed. 



Views with regard to the alteration of peridotites and allied olivine 

 rocks to serpentine have changed considerably in recent years. It was 

 formerly thought that the alteration was brought about by the processes 

 of weathering, but now it is quite generally believed to be deep-seated.*^ 

 Heated waters probably following closely upon the intrusion of the 

 magma itself and given off by it during solidification, it is thought, have 

 brought about the alteration of the olivine to serpentine while still buried 

 at considerable depths. 



Hornblende Porphyrite 



A dike of hornblende porphyrite crosses the granites and pegmatites 

 in a large cut just north of Springdale, about four and one-half miles out 

 of Stamford on the New Canaan branch of the New York, Xew Haven 

 and Hartford Railroad. As this is the only occurrence of a basic in- 

 trusive which is clearly of later age than the granitic intrusives in the 

 area studied, a brief description of it will not be out of place. 



The dike is about three and one-half feet thick at its widest part but is 

 quite variable. The strike of the dike is about N". 48° E., and the dip 

 is practically vertical. In a hand specimen, it shows a felsitic texture 

 and dark green color. When examined in thin section under the micro- 

 scope, the texture is apparently ophitic, but the space between the feld- 

 spar laths is occupied by hornblende instead of augite. The rock con- 



*i Ernst Weinschenk : Allgemeine Gesteins-kunde als Grundlage der Geologic, pp. 

 119-121. 1902. 



