356 G. H. Williams — Hornblende and its Gliding Planes. 



When viewed between crossed Nicol prisms, a section like 

 that shown in fig. 2, displays the twinning lamellae with a dis- 

 tinctness equal to that of calcite. The obtuse bisectrix, c, is 

 inclined to the vertical axis, as indicated by the cleavage lines, 

 about 11°. It also lies in what is usually considered the acute 

 angle ft for hornblende ; and hence, while the cleavage lines of 

 the twinning bands and the principal crystal are inclined ca. 

 32° to each other, their extinction directions make an angle of 

 only 10° (fig. 4). This optical orientation is in entire accord 

 with the measured angles as indicating Poo (101) to be the 

 twinning plane for these intercalated lamellae. 



Aside from the intrinsic interest attaching to the discovery 

 of secondary transverse twinning lamellae in hornblende, iden- 

 tical in character with those already well known in pyroxene, 

 these lamellae and their attendant parting planes are important 

 indications of the correct crystallographic orientation of these 

 two closely related minerals. The positions now generally em- 

 ployed for crystals of hornblende and pyroxene are arbitrary 

 while they lack uniformity. I shall attempt to briefly indicate 

 the reasons why the position commonly selected for hornblende 

 should be so altered as to make its present orthodome Poo 

 (101) the basal pinacoid. No such important change should 

 be advocated without abundant evidence in its favor, inasmuch 

 as itnnodifies a long established usage on a common mineral. 

 Nevertheless, if this usage does violence to natural relation- 

 ships, such a change is warranted. 



As is well known, both pyroxene and hornblende possess 

 two planes in their orthodiagonal zones which have nearly the 

 same inclination to the vertical axis. Assuming that the 

 position usually adopted for pyroxene is correct, since it makes 

 so important a direction as the gliding plane the basal pinacoid, 

 then the arguments for reversing the common usage for horn- 

 blende are of four kinds, which are based upon 



1. relative inclinations of the planes, 



2. optical orientation, 



3. structure planes, 



4. parallel growths. 



1. As they are now usually placed, the steeper of the two 

 nearly equally inclined faces of the orthodiagonal zone is made 

 the base on pyroxene, while on hornblende the flatter of the 

 two is thus employed. Hence, if the positions of orthodome 

 and basal pinacoid were reversed on hornblende, there would 

 be uniformity established in this respect. 



2. With the present orientation, the axis of elasticity c, which 

 is nearest to the vertical axis, lies with pyroxene in the obtuse, 

 but with hornblende in the acute angle ft. If, however, such 

 an alteration as that above mentioned were made, there would 

 also be uniformity established here. 



