M ELI LITE- NEPHELINE -BASALTS. 345 



clinic augites. One of them exhibits a violet-gray color in thin 

 section and belongs to the basaltic augites; the other one becomes 

 transparent with a dark green color. Both form numerous phen- 

 ocrysts, but the first occurs somewhat more frequently. They 

 occur as single crystals and are also grown together in a zonal 

 manner, the green one always forming the center, the gray one 

 the outer parts of the crystals. Hence the gray augite is the 

 younger. The pyroxene in the groundmass shows the same 

 color and properties. The pleochroism of the two minerals is as 

 follows: 



Gray augite. Green augite. 



a Brownish-yellow Light yellowish-green 



6 ^ ,;.. , ^ Dark g"ray-green. 



\ Violet-gray ta i -^ 



C ^ ^ ■' Dark green. 



The angle of extinction, c: c, is large and, as may be seen in 

 the zonal crystals, it is somewhat larger in the gray pyroxene 

 than in the green. The extinction in sections cut approximately 

 parallel to (oio) has been observed to be about 47 degrees (gray 

 augite) and 41 degrees (green augite). The two pyroxenes show 

 in addition to the cleavage parallel to (no) another but less 

 distinct one parallel to (oio). Inclusions of magnetite, apatite 

 and glass are common. 



Phenocrysts of feldspar are scarce. In part they show the poly- 

 synthetic twinning lamination of plagioclase; in part the latter is 

 wanting and one of the latter feldspars, which was isolated and ex- 

 amined for specific gravity and optical properties, was found to be 

 sanidine. Phenocrysts of nepheline are more frequent than those 

 of feldspar. The mineral appears partly in the form of short-pris- 

 matic crystals, partly in rounded grains. It presents distinct 

 cleavage, parallel to (10 10) and to (oooi), and the usually ob- 

 served optical properties. Isolated grains are decomposed by 

 hydrochloric acid with the separation of gelatinous silica; the re- 

 sulting solution when evaporated gives numerous cubes of NaCl. 

 Inclusions are scarce; there are fluid cavities with moving bub- 

 bles, generally arranged in rows, besides some pyroxene crystals. 



Apatite forms short and stout crystals always filled with in- 



