272 REGINALD E. HORE 



brown pyroxenes. Biotite and black iron oxides are generally visible. 

 Quartz is generally present; but often in small quantity intergrown 

 with feldspar and not always visible to the naked eye. In some speci- 

 mens there is a decided pink color due to the presence of pink feldspar, 

 and in these portions quartz is more prominent, and grains of pyrite 

 and chalcopyrite are frequently visible.. In rarer instances the rock is 

 almost completely made up of feldspar and quartz with but small 

 amounts of ferromagnesian minerals. In some of the masses there 

 are numerous small aplitic veins closely related to the quartz-feld- 

 spar portions just mentioned but practically free of ferromagne- 

 sian minerals. 



Microscopic character. — Specimens of medium-grained gray dia- 

 base from all parts of the district show very similar composition. 

 Plagioclase feldspars and pyroxenes are the only minerals present in 

 large quantity. Iron ores are always present in small quantity, 

 and filling the interstices is a micrographic intergrowth of feldspar 

 and quartz. 



In thin sections the fresh rock is nearly colorless. Generally the 

 feldspar is somewhat clouded with alteration products, and the pale- 

 brown tinted pyroxene has usually some greenish uralitic or chloritic 

 spots. Angular particles of black ilmenite are generally partially 

 altered to grayish-white leucoxene. 



Other minerals sometimes found include biotite, hornblende, 

 quartz, apatite, olivine, chalcopyrite, and pyrite. Among secondary 

 minerals are chlorite, sericite, leucoxene, sphene, limonite, epidote, 

 and carbonates. 



Pyroxene. — The common pyroxene is of very pale brownish color. 

 Rarely one finds decided violet-tinted varieties. The shape is irregu- 

 lar and has evidently been determined by the nature of the spaces 

 left after most of the feldspar had crystallized. Less commonly 

 the pyroxene and feldspar crystal boundaries are equally well 

 developed. The size varies greatly in slices from different specimens; 

 commonly they are from 2 to 5 mm. but some of coarser grain show 

 blades of pyroxene from 5 to 20 mm. in length. These large crystals 

 are usually twinned and show marked diallagic striae and they are 

 generally somewhat altered to a greenish fibrous hornblendic sub- 

 stance. Some slides show numerous grains which have the optical 



