G. H. Williams — Peridotites near JPeekskill, 2V. Y. 31 



scale in the coarse grained olivine-gabbros of the same district.' 54 " 

 Professor M. E. Wads worth in a recent description of the 

 Lherzolite from Presque Isle, Michigan, alludes to the same 

 appearance and suggests that the reflecting mineral plays the 

 same role in the peridotite that the iron does in the pallasites.f 



In all cases heretofore described, the reflecting, bronzy min- 

 eral appears to have been some variety of pyroxene — either 

 augite, diallage or enstatite ; in the peridotite from King's 

 Ferry, however, the glistening surfaces, as the microscope 

 shows, are those of a brown hornblende.^ The individuals of 

 this mineral are very large, being often four inches in diameter ; 

 but, notwithstanding that they are so abundant as to be every- 

 where in contact with each other, so full are they of inclusions 

 of the other constituents that they do not together make up 

 one-half of the entire mass of the rock. (No. 90.)§ 



Another variety of this rock from the same locality, in which 

 the reflecting surfaces of hornblende are considerably smaller, 

 is much fresher than the one just described. (No. 95.) 



A macroscopical examination of these rocks is able to dis- 

 close, in addition to the ingredients already mentioned, only 

 frequent particles of magnetic pyrites (pyrrhotite) and glisten- 

 ing flakes of a light green mica ; a microscopical study of them, 

 however, reveals much that is interesting. 



The Hornblende is undoubtedly the most important and 

 characteristic constituent of this group of olivine rocks, for it 

 is to this mineral that their peculiar habit is almost wholly 

 due. That it is really hornblende which is present and not, as 

 might be supposed by analogy with similar occurrences, some 

 variety of pyroxene, is proven by the cleavage angle. Several 

 measurements on a large Fuess reflection- goniometer gave 

 values varying between 124° 15' and 124° 50' (calculated angle 

 betweeen the prism (oo P) faces for hornblende, 124° 3CK). 



When examined under the microscope by transmitted light 

 this mineral appears of a rich brown color, belonging to the 



* The Copper bearing Rocks of Lake Superior. Monographs of the TJ. S. Geol. 

 Survey, vol. v, 1883, p. 42. 



f Lithological Studies. Part I. Mem. Mus. Comp. Zool. of Harvard College, 

 vol. ix, p. 136, 1884. 



t Professor Bonney has recently described a peridotite from Swift's Creek, 

 Gipps' Land, Australia, which very strongly resembles both the Schriesheim and 

 the Stony Point rock. The large cleavage surfaces with an interrupted luster 

 Bonney determined to be those of hornblende. This mineral had a green color 

 and only a weak pleochroism. It is considered by the author to be possibly of 

 secondary origin having been produced by the paramorphism of the pyroxene. 

 (See Mineralogical Magazine, vol. vi, p. 54, July, 1884.) 



§ The numbers given in connection with the different specimens are those of 

 the collection of Cortlandt rocks belonging to the Johns Hopkins University. In 

 case a letter is attached to a number, W". indicates that the specimen or section in 

 question belongs to the writer's private collection, while D. refers to sections 

 loaned by Professor James D. Dana, for whose kindness in this respect the writer 

 would express his deep obligation. 



