62 MINERALOGY AND LITHOLOGY. 



determined by the lower Nicol, which alone is on the microscope, is 

 indicated by the arrow, and it is seen that, when the light passes through 

 a basal section parallel to the orthodiagonal axis, the crystal is green, 

 and, through a like section parallel to the clinodiagonal, it is bright yel- 

 low, while a prismatic section, with the vertical axis parallel to the plane 

 of vibration of the light, is blue. With ordinary light, the predominant 

 color of these crystals is green, only varying in shade because blue and 

 yellow make green, and green and yellow make green, as do also green 

 and blue. In this figure it will be noticed that the crystals are not 

 terminated, while the prismatic faces are well developed. This is quite 

 characteristic of ordinary hornblende in the rocks. The basaltic horn- 

 blende shows a very strong absorption of the light, rather than a marked 

 pleochroism. The characteristics given distinguish hornblende very well 

 from augite, which is not pleochroic, is right-angled in its cleavage, and 

 very different in crystalline outline. 



When a section of hornblende is revolved between crossed Nicol 

 prisms, the interference colors are not bright, especially in the basaltic 

 varieties. Bright colors are obtained, however, with the lighter colored 

 kinds. As with augite, sections containing the orthodiagonal axis are 

 dark between crossed Nicols when a crystallographic axis falls with the 

 plane of vibration to the light. With other sections, this is not true ; 

 and a section cut parallel to the clinopinnacoid, and placed with the ver- 

 tical axis parallel to the plane of vibration of the light, must be revolved 

 15° before it becomes dark, showing that one elasticity axis makes an 

 angle of 15° with the crystallographic vertical axis. 



It has been shown that augite and hornblende are frequently associ- 

 ated together in our rocks as a result of alteration ; but it is also true 

 that at times they are associated together in the same rock, apparently 

 both being simultaneous formations. 



It has been pointed out, first by G. Rose, that these two minerals are 

 referable to the same fundamental form ; that is, if the prismatic planes 

 of pyroxene making an angle of 87°. 5 are called I, then the plane i-2 will 

 correspond to the I of hornblende. In other words, the orthodiagonal of 

 the hornblende crystal has twice the length of that of pyroxene. This 

 makes the two minerals isomorphous in form ; but since the minerals 

 have different cleavages and habits of crystallization, they must still be 



