//. Williams— Homblendi audits Gliding Planes. 



occur- most abundantly near Pierrepont Their chemical simi- 

 larity may be gathered from the preceding analyses: I being 

 an average of two analyses of the green variety made by Dr. 

 \Y. M. Burton formerly of the Johns Hopkins University; 

 and II. an analysis of the brown variety made by Dr. T. M 

 Chatard of the u. 8. Geological Survey, 



In babit these hornblende crystals are usually short and Btout, 

 though sometimes much elongated. Their terminal planes 

 arc few and rough, while the faces of the prismatic zone are 

 numerous and glistening. Indeed, certain o\ the green crystals 

 from Russell possess an unparalleled development of prismatic 

 forms, showing six faces between the pinacoids whose symbols 

 are as follow-: »P7 (170), »Ps (150), »Ps (130), »P (110), 

 »P2 (210), xP3 (310).* ' Of these forms, all except the first 

 have also been identified on the brown crystals from Pierrepont. 



Ir was on certain small brown hornblende crystals of simple 

 prismatic habit, xP (HO) and ooPqo(OIO), from South Pierre- 

 pont that the transverse parting with parallel twinning 1am- 

 ellffi was first observed. Subsequent search for more material 

 -hewed that, although the parting itself occurred quite fre 

 quently on both the brown and green hornblende from northern 

 New Y<>rk, twinning lamellae parallel to the parting, which 

 were of sufficient breadth to allow of their optical orientation, 

 were apparently confined to such simple brown crystals as 

 those above mentioned. f 



( rv-tals of both the green and brown hornblende, where the 

 parting is present without visible twinning lamellae, often yield, 

 on separating, a surface of high luster which allows of an exact 

 location of the parting plane. A number of measurements of 

 its inclination to the faces of the fundamental prism, clearly 

 showed this plane to be parallel, not to the form usually as- 

 sumed as the basal pinacoid for hornblende, as at first surmised 

 by vom Rath and myself, but to the unit orthodome, Poo (101), 

 which has nearly the same inclination to the vertical axis. 

 The angles obtained on one large crystal of brown hornblende 

 from South Pierrepont against the four faces of the unit prism 

 are as follows : 



101 : 110 = 104° 12' ) ,^- „, na „ , , „ , . . 



101:110 = 104° 10' [ 1047 26' (calc. v. Koksch.) 



- . o 



101 : 110 = To" 49 

 lOl : 110 = 75° 40 



., j- 75° 52' 34' (calc. v. Koksch.) 



* Neues Jahrbuch fur Min., etc., 1S8."3. II. p. IT.";, where this hornblende 

 wrongly designated as pargasite. 



f For their kindness in willingly furnishing hornblende material for this inves- 

 tigation, the writer desires to expn obligation to the following gentlemen: 

 J. H. Caswell. Esq., of New York; Messrs. C. B. Benient and Joseph Wilcox of 

 Philadelphia. C. I). Nims of St. Lawrence County, X. V . 1'r. Whitman I 

 Washington. Prof. S. L. Penfield of New Haven, and the authorities of the I 

 Geological Survey 



