G. H. Williams — Peridotites near Peehshill, 1ST. Y. 35 



ranged parallel to one or more of the crystallographic axes of the 

 olivine, although they are sometimes more irregular in their 

 distribution. Frequently these rods, instead of being straight, 

 are variously bent and twisted, exhibiting the forms of trichites 

 in obsidians. In such cases they show a tendency to form 

 elliptical groups resembling a fine arabesque, as figured by 

 Zirkel. The same author has observed that while these inclu- 

 sions are very characteristic of the olivine of the older rocks, 

 they are never found in that of the younger basalts. I 

 could not see that those occurring in the Cortlandt peridotite 

 were ever translucent as stated by Zirkel and Rosenbusch for 

 other localities. There seems little doubt that they are com- 

 posed of magnetite, since they are readily decomposed by acid, 

 and since such grains of olivine as contain them in abundance 

 are attracted by the magnet. 



Aside from the ordinary alteration of olivine to serpentine, 

 which may be most instructively studied at every stage in the 

 Cortlandt rocks, the most interesting phenomenon exhibited 

 by this mineral is the beautiful development of reactionary 

 rims or zones, wherever the olivine comes in contact with 

 feldspar. This latter mineral is indeed no essential ingredient 

 of the peridotites, but as already mentioned, they constantly 

 show a tendency by its assumption to grade into olivine-gab- 

 bros and olivine-norites. Wherever olivine comes in contact 

 with feldspar, no matter how fresh both of the minerals may be, 

 there is always present between 

 them a double zone, the inner por- 

 tion, nearest the olivine, being com- 

 posed of square grains of nearly 

 colorless pyroxene and the outer 

 one of tufts of radiating actinolite 

 needles of a beautiful bluish-green 

 color and strongly pleochroic. Cer- 

 tain slides in Professor Dana's col- 

 lection from Stony Point show this 

 structure in great perfection, (see fig- 

 ure). The interior band of pyroxene 

 is here O'O? mm. wide ; the exterior one 0*15 mm. The same phe- 

 nomenon has been described by Tornebohm in the olivine-hyper- 

 ites from Olme in Sweden* and is even more wonderfully devel- 

 oped in a coarse grained olivine-norite from the south shore of 

 Lake St. John, Prov. Quebec, Canada.f So constant is the de- 

 pendence of this zone upon the contact of the olivine and the 

 feldspar, that it must be in some way due to a reaction between 

 the substance of these two minerals, the resultant amphibole and 



*Neues Jahrbuch fur Min., etc., 1877, p. 383. 



f Via. F. D. Adams, Am. Naturalist, Nov., 1885, p. 1087. 



