354 G. II Williams — Hornblende and its Gliding Planes. 



This result is quite in accord with the position of a similar trans- 

 verse parting as determined by Cross* for hornblende in cer- 

 tain dioritic rocks from Brittany, and by Miiggef for analogous 

 cases from Arendal. 



Upon the simpler crystals from South Pierrepont, which 

 show distinct twinning lamellae parallel to the parting, this 

 plane is itself always dull, so that no certain result can be 

 obtained from its reflections. That its position, however, is 

 the same as that of the above determined plane, Pa^(lOl), is 

 proved both by the optical orientation of the lamellae and by 

 the angle between the prismatic faces of the lamellae and those 

 of the principal individual. The appearance of such a crystal 

 is represented in fig. 1 ; and the transverse lamellae, although 



much narrower than those here depicted, are broad enough to 

 yield distinct prismatic reflections. These gave the angles: 



(110) : (110)' - 147° 38' 

 (110) : (110)' — 147° 44' 



while 147° 56' 26" is required by von Kokscharof on the sup- 

 position that the twinning axis is the normal to P56~(101). 



These narrow twinning lamellae parallel to the transverse 

 parting in the Pierrepont hornblende vary in width and are 

 not always continuous across the crystal. Their maximum 

 breadth is less than 0*04 mm , but under the microscope they 

 nevertheless come out with a distinctness which allows of their 

 complete optical orientation. The appearance of a portion of 

 a thin section, cut parallel to the clinopinacoid and magnified 

 about eighty diameters, is represented in fig. 2. In ordinary 

 light the twinning lamellae appear as a series of parallel bands, 



* Tschermak's Min. und Petrogr. Mitth., Ill, p. 386, 1881. The first observation 

 of such a transverse parting in hornblende was made, as far as I know, by Jere- 

 mejew in 1872. Neues Jahrbuch fur Min., etc, 1872, p. 405. 



f Neues Jahrbuch fur Min., etc., 1889, I, p. 243. 



