338 GEOLOGY OF THE EUREKA DISTRICT. 



pericline law. The decomposition commenced at the center, resulting in some cases 

 in a cryptocrystalline aggregate like kaolin, with calcite; in others filling the crystal 

 with shreds of colorless mica, and minute, pale yellow grains, traceable to larger 

 aggregations of epidote. Feldspar and quartz" form the main mass of the rock, 

 through which is scattered mica and hornblende in varying amounts. The hornblende 

 is in poorly defined crystals, except some of the smaller individuals, which are well 

 developed in the prism zone. The prismatic faces are much larger than the clinopina- 

 coid, and the cleavage parallel to the former is strongly marked. It is in simple 

 crystals and twins, twinned parallel to ooP do. The color is dark green, with strong 

 pleochroism, most noticeable in sections parallel to the clinopinacoid and base. The 

 colors are: c = dark green, b = brownish green, a = light brown, c = b > a. The angle 

 of extinction read from the vertical axis is mostly from 17 to 19, but in two instances 

 is 21 and 25. It incloses magnetite, apatite, and biotite, having been formed 

 after the latter in every case. It is quite fresh, though the mica is almost com- 

 pletely decomposed. The biotite, with which the hornblende is intimately asso- 

 ciated, occurs in comparatively thick crystals of irregular outline, of a deep brown 

 color, with nearly uuiaxial interference figure, and has occasional inclusions of iron 

 oxide, apatite, zircon, and rarely feldspar. It is especially interesting from its mode 

 of decomposition, which takes place along the basal cleavage and results in a dark 

 green pleochroic chlorite, which must be formed of an aggregation of -minute scales 

 parallel to the lamination of the mica, for basal sections remain dark when revolved 

 between crossed nicols and show no interference figure and no pleochroism, while 

 transverse sections exhibit a marked fibration parallel to the mica cleavage and are 

 pleochroic ; being green, parallel, and yellow at right angles to the line of fibration. 

 This chlorite, in turn, alters into epidote and possibly quartz. The epidote, in irregu- 

 lar grains, is pleochroic between intense greenish yellow and pale yellow. That it 

 does not result directly from the decomposition of the biotite is evident from the fact 

 that it never occurs in it unassociated with chlorite, while the latter occurs constantly 

 alone, and also because lenticular masses of epidote are seen to have disturbed the par- 

 allelism of the chlorite scales, proving its subsequent crystallization. 



Titanite, in narrow rhombic sections and less regular grains is sparingly present. 

 The iron oxide appears to be magnetite for the most part. Colorless apatite is 

 abundant both in short, stout prisms, and long, slender, jointed needles, penetrating 

 everything in all directions. Apatite and sharply crystallized zircon appear to be the 

 first minerals formed in the rock. In thin section 2 there are three comparatively 

 large crystals of allanite, dark brown, with strong absorption; two are twinned. An 

 irregular grain of allanite is found in 4. Thin section 3 is highly decomposed and 

 stained with hydrous oxide of iron. Thin section 4 is of a porphyritic variety, having 

 a fine grained, microgranitic groundmass of quartz and feldspar. Though the feldspar 

 of this rock is still mostly fresh, and the hornblende entirely so, the biotite is com- 

 pletely altered to green chlorite, epidote, quartz, and calcite. 



