'/. //. William* — HornMend and its Gliding Planes, 855 



across which the cleavage lines are inclined 32 to those of the 

 main crystals. V< r\ ..t'tm a thin film of calcite lies oil one 

 side of the twinning band; and. in Borne cases, this may be 



i r-» cross from the lower to the upper Bide, as Indicated in 

 fig. 2. The width of these bands is ?ery variable, as is also 

 that of different portions of the same hand. Within the born- 

 blende are irregularly shaped intercalations of another mineral 

 >mewhat darker hrown color than the hornblende and \ 

 with a parallel extinction. It is a mica, probably phlogopite, 

 which i> regularly grown with the hornblende in such a way 

 that their cleavages are parallel : although, in some cases, ir> 



ition is wholly independent of that of the hornblende. 

 Across large areas of this mica the twinning lamella do not 

 pass, hut they have evidently created more or less disturbance 

 in the mineral where they could not produce a gliding (fig. 2). 



try narrow bits of the mica are, however, displaced by the 

 gliding of the hornblende (fig. -°>). 



In some cases the lamellae die out gradually in the horn- 

 blende, while in others they terminate abruptly at a crack, 

 beyond which they may he continued at a different level. 

 Their microscopic character bears strong witness to the sec- 

 ondary and dynamic origin of these twinning bands. Within 

 them the cleavage cracks are much more abundant than in the 

 undisturbed hornblende substance. Along their edge the 



3. 4. 



hornblende is slightly frayed out and bent, as though by a 

 forcible shoving ; and, in certain parts of the lamell®, the 

 little fibers are twisted across the whole width of the hand, bo 

 as to produce an irregular or undulatory extinction. In >till 

 other- ilong the line of a fissure, small parts of the hand 



arc displaced from their normal direction, as Bhown in tig. 2. 



Pig. 3 BhoWB the highly magnified detail of lamelhe in their 

 passage of a mica area and crack. 



