36 G. II. WUliams — Peridotites near PeeJcskill, N. T. 



pyroxene having an intermediate composition. So sharply de- 

 fined, however, are the crystals of this zone against the per- 

 fectly fresh feldspar and olivine substance that it is difficult to 

 conceive of them as produced after the rock had entirely solidi- 

 fied. They may have been formed by a reaction between these 

 substances while at least one of them — the feldspar — was crys- 

 tallizing, although in some cases the formation of the actinolite 

 seems to have continued after this time. In any event, all 

 traces of this border around the olivine disappears the instant 

 this mineral comes in contact with any other constituent than 

 the feldspar. 



The pyroxene constituent of the peridotite from King's Ferry 

 appears to be for most part hypersthene. It sometimes forms 

 small irregular grains not larger than those of the olivine, but 

 in other specimens it is present in individuals which are over a 

 centimeter in length, enclosing the smaller grains of both oli- 

 vine and hypersthene like the hornblende. In all forms it 

 possesses all the ordinary characteristics of hypersthene, ex- 

 cept that it is singularly free from the usual inclusions. Its 

 pleochroism is very strong : a = a ray, red ; b = b ray, yellow ; 



c = c ray, green. Its cleavage is well developed parallel to 

 the prism (oo P) and also still better parallel to the brachypina- 

 coid (oo P ob ). Its orthorhombic character is proven by its 

 parallel extinction and the appearance of a bisectrix when such 

 sections as are cut nearly perpendicular to the vertical axis are 

 examined in converging polarized light. A common, non-pleo- 

 chroic augite, in which a cliallage habit is frequently developed 

 by the presence of a parting parallel to the orthopinacoicl, is 

 also often to be observed in this rock, although in many speci- 

 mens it is altogether lacking. As this constituent increases in 

 importance the rocks grade into those of the next group. 



The only remaining silicate which enters into the composi- 

 tion of the hornblendic peridotites is the biotite. This mineral 

 rarely retains its brown color. It is generally so completely 

 bleached as strongly to resemble muscovite in thin sections. 

 It is much bent and twisted, often having small lenses of cal- 

 cite interposed between its lamella?, like those figured by Dr. 

 Hussak.* Aside from mere bleaching, the formation of the 

 bright green, chloritic mineral, noticed as an alteration product 

 of the hornblende, is also frequent. The true character of this 

 mica is revealed by its very small axial angle — the hyperbolas 

 being hardly seen to open at all when cleavage pieces are ex- 

 amined in polarized light— as well as by the fact that rarely 

 sections may be found which have escaped the bleaching. 



* Anleitung zum Bestiinmen der gesteinbildenden Mineralien, 1885, Taf. Ill, 

 fig. 81. 



