MINERALOGY. 57 



all Other sections will show a variation from this deportment, and if a 

 clinodiagonal section (r) is examined it will be found that if the vertical 

 axis is placed parallel to the plane of a Nicol prism, the section must be 

 revolved 39° (or the complement of this angle in the other direction) in 

 order to reach the point where the section will be dark between crossed 

 Nicols, and not distort a calcite interference cross ; thus showing that 

 an elasticity axis makes this angle with the vertical crystallographic 

 axis. 



Augite is more often in grains, showing no crystalline faces, and then 

 its cleavage serves to distinguish it from other minerals with which it is 

 liable to be confounded ; and it is to be noted that the mineral is not 

 markedly dichroic, and hence when revolved with simply the polarizer on 

 the microscope, no marked variation in color is seen, as it comes into 

 different positions with reference to the light ; but when revolved be- 

 tween crossed Nicols, the interference colors that are obtained are very 

 brilliant. 



Pyroxene is sometimes foliated, the laminae being parallel to the ortho- 

 pinnacoid. Pyroxene of this structure characterizes the rock gabbro. 



The alterations that augite undergoes, as developed by microscopic 

 study, are quite interesting. The most evident one in our rocks is the 

 alteration of augite into hornblende, of which examples are common. This 

 change is merely a molecular one, since the two minerals have the same 

 composition ; but it becomes very evident by the alteration of cleavage, 

 and all other physical characters. Sometimes the change is complete, 

 giving us hornblende in augitic forms; and sometimes it is partial, when 

 we have hornblende with a core of augite. This change was first noted 

 by Gustav Rose, who named the hornblendic product uralite, and the 

 rocks containing it uralite porphyry, &c. The resulting uralite has 

 usually a fibrous structure. I have never seen so pretty an illustration 

 of this kind of change as is furnished by the augite sienite of Jackson, 

 in our state. Here the augite is not altered into a fibrous green uralitic 

 mass, but into fine, compact brown hornblende, which contains, as a rule, 

 a core of augite. The cleavages of the two minerals also bear a definite 

 relationship to one another. If we lay out the lateral axes of a crystal of 

 augite (see PI. 7, Fig. i) and connect their ends, we shall have a nearly 

 square figure, which is the base of the prism of augite, and the sides of 



VOL. IV. 8 



