62 
ALASKA GEOLOGY 
the region, and later charged with gold-bearing pyrite by 
mineralizing solutions. 
The sodium-syenite consists essentially of almost pure 
albite, in granular or partly idiomorphic aggregates, with 
which is more or less orthoclase and quartz, the latter 
generally in micropegmatitic intergrowths filling the in¬ 
terspaces of the albite. Ferromagnesian constituents were 
probably present in the unaltered rock, but their nature 
can not now be determined, beyond the fact that there 
was a small amount of biotite, now changed wholly to a 
greenish mica or to chlorite. Other original accessory 
constituents are apatite and titanite, fairly abundant, and 
sparing zircon. The secondary products are, first and 
most important, pyrite, which is abundant in all slides in 
sharp crystals, often surrounding small feldspar crystals 
and often associated with calcite. Calcite and sericite 
are very abundant, being developed especially in the cen¬ 
ters of albite crystals, which they sometimes completely 
honeycomb, leaving only a narrow rim of unaltered feld¬ 
spar. Grains of epidote and zoisite are sometimes abun¬ 
dant in the feldspar, and rutile in sagenitic groups is seen 
in the chloritized biotite. Quartz in evidently secondary 
forms is surprisingly small in amount, although this is ac¬ 
counted for partly by the choice of material for slicing, 
which was the freshest obtainable. It should, however, 
be said that sections from rocks which were pronounced 
by those most familiar with the ores to be undoubtedly 
gold-bearing, and sections made from portions of the 
syenite shown by assay to be barren, were practically 
identical in character, there being but little observable 
difference in decomposition or in content of quartz or 
pyrite. Hand specimens of the barren rock usually 
showed a darker color, due to the greater amount of 
chlorite, but this distinction was not constant. In fact it 
was continually a matter of surprise that the syenite was so 
