G. II. II illtams — Hornblend< <iml its Gliding PUtm 



The practical identity in character of the gliding planes ;m<l 

 mdarv lamella* in the case of hornblende and pyroxene hae 

 already been emphasized ; and yet, with the present usage, we 

 must make one of these parallel to IV (101), and the other 

 parallel to OP (001). The importance of being able to assign 

 to them the same crystallographic symbol is at once apparent, 

 and it has already been insisted upon by ft£ugg< 



4. Much the strongest argument for reversing the Bymbols of 

 the orthodome and basal pmacoid of hornblende is. however, 

 to be derived from nature's usage. As far as I have been able 

 to extend my observations, all the many parallel growths of 

 pyroxene and hornblende, whether original or produced by 



ondary paramorphism, are such that the two species have 

 their vertical and orthodiagonal axes in common, while the 

 basal plane of the former is as nearly as possible parallel to the 

 orthodome of the latter. Thi> certainly indicates the crystal- 

 lographic homology of these two faces, and is alone ample 

 • ii for giving them the same symbol. 



Loin: before gliding planes had been recognized on either 

 pym\ene or hornblende, vom Rath described and figured par- 

 allel growths from Vesuvius like those above mentioned, f and 

 e them as sufficient ground for reversing the accepted sym- 

 bols <»f one of the two minerals. He suggested that the 

 change be made on pyroxene, but in this he has not been fol- 

 lowed, nor should he be, since it is desirable to keep so 

 important a face as the gliding plane for the basal pinacoid. 



1 have examined a large number of parallel growths of 

 pyroxene and hornblende in rocks, and have found the lesser 

 extinction angle for both minerals to lie on the same side of 

 the vertical axis. This indicates an orientation similar to that 

 observed by vom Hath. 



In this connection some of the material from St. Lawrence 

 County is particularly instructive. The brown hornblende 

 from Pierrepont is associated with pale green salite and white 

 albite of a lenticular habit. Parallel growths of the horn- 

 blende and salite are not uncommon. Fig. 5 represents a thin 

 section of such a growth, cut parallel to the clinopinacoid, 

 which i> common to both minerals. The section shows that 

 the vertical axes of both are also parallel, while the position of 

 the extinction directions proves that the planes commonly 

 known as the basal pinacoid for pyroxene and as the ortho- 

 dome for hornblende, are nearly parallel. In the former min- 

 eral the transverse parting and twinning arc well developed. 

 In this specimen of hornblende these arc absent, but we can 



* Xeues Jahrbuch fur Min.. etc.. L889, i. ]». 244. 



f Verhandl. Naturliist. Vereina d. preuss. Rheinl. u. Westfalens xx.viv, 6 Folge. 

 Vol. iv. Xeuos Jahrb. fur Min.. etc.. isTG. p. .'5!M. pi. viii. Bg& 6 am 



