MINERALOGY. 9I 



much larger angle than any other feldspar, the angle for anorthitc being 

 from 27° to '^'j°, through which it must be turned in order to produce 

 in either of its sets of laminas a maximum of darkness. 



If, now, a section of one of these fine anorthite crystals is cut, nothing 

 of the kind is to be seen. The section placed between crossed Nicols 

 appears made up of the most immense number of minute particles; the 

 field is uniformly light ; and, on revolving the section, no effect whatever 

 is produced, and no point of maximum darkness is obtained. In other 

 words, these crystals possess aggregate polarization, and are no longer 

 feldspar ; but the large crystal is made now of an immense number of 

 little ones lying in every possible direction. I analyzed this feldspar pro- 

 duct, with the following result : 



Typical 

 anorthite. 



43.10 

 36.90 



99.72 100.00 



Here is seen the progress of a change, which may result in the conver- 

 sion of a basic into an acidic rock. Lime is removed, alkali is gained; 

 and, while other minerals decompose, losing all their bases, and leaving 

 residues of silica, here we see how this most basic feldspar is approach- 

 ing to that composition, which, with the addition of some silica and a 

 recrystallization, can form the orthoclase of our granites. 



Now, smaller crystals of this same feldspar in the rock are not so en- 

 tirely decomposed ; and as the case is instructive, and the decomposition 

 so prettily seen in progress, I add a drawing of a section of one of the 

 crystals, which is seen in Fig. 3 on PI. 5. The outside of the crystal is 

 entirely altered into an aggregate, while within the fresh material shows 

 the bands characteristic of triclinic feldspars. On revolving this section 

 between crossed Nicols, the feldspar within shows alternations of color 

 and darkness, and its optical properties can be determined, while the 



