PERIDOTITE DIKES NEAR ITHACA, N. Y. 



267 



altered to serpentine and a carbonate. None of the phenocrysts con- 

 tain inclusions of magnetite, ilmenite, or perofskite, except such fine 

 grains of magnetite as commonly result from the alteration of the 

 olivine. The groundmass consists chiefly of serpentine with some 

 mica, and small amounts of magnetite, ilmenite, and a carbonate. 

 Much of the mica of the groundmass has lost its original color, and 

 become either green or light brown. Flow structure is very common. 

 The porphyritic phase of the peridotite is confined to the small dikes 

 and to the borders of the large ones. 



The non-porphyritic peridotite was originally composed of olivine 

 and biotite, with subordinate amounts of diopside, magnetite, ilmenite, 

 perofskite, picotite, and apatite. Much of the olivine has been 

 changed to serpentine with the formation of magnetite, opal, and a 

 carbonate. The pyroxene is partly altered to calcite and serpentine. 

 The ilmenite and titaniferous magnetite have been partly removed 

 by solution, leaving a white, cloudy substance, leucoxene. The 

 perofskite has partly changed to calcite and rutile. The relations 

 of the minerals indicate that apatite, ilmenite, magnetite, perofskite, 

 and picotite all belong to an early stage of crystallization; following 

 these came the diopside; and still later the mica and olivine. The 

 mica and olivine were in part pyrogenic, as indicated by their mutual 

 interference during growth. 



Mineral composition of the rock. — The following tables give approxi- 

 mately the proportions of the different minerals present, as determined 

 by microscopie measurement; 



Serpentine 



Biotite 



Olivine 



Ilmenite and magnetite 

 Perofskite and picotite. 



Apatite 



Calcite 



Diopside 



99.918 



99.817 



The section for No. I was taken from near the center of the Glen- 

 wood dike, while the one for No. II was taken from near the margin 

 of the same dike. In both cases the rock is considerably altered. 



