MUSEUM OF COMPARATIVE ZOOLOGY. 183 
Both microcline and probably albite have also formed independently 
of any pre-existing nucleus, as far as their outline gives my clue; but in 
other cases, if the facts are correctly interpreted above, the albite areas 
owe their position to replacement and accompanying enlargement of 
detrital feldspar cores of undeterminable species. 
It has been stated that this conglomerate of Bear Mountain passes 
into an albite schist in places, and that the Cambrian conglomerate of 
Hoosac Mountain also passes into a similar rock conformably at the top 
of the conglomerate series. The nature of the feldspar in the Hoosac 
rock and in the similar schists westward in Greylock Mountain has been 
proved by analysis. 
In the slides of several specimens from the latter region the albites 
show an apparent enlargement, not uncommon in the albites of albite- 
phyllites in general. These albites are in large rounded grains, either 
simple crystals or singly twinned. Each crystal polarizes homogene- 
ously, having a very ragged outer edge. They contain a black mate- 
rial disseminated through their substance, which is apparently black 
oxide of iron mixed with graphite; this substance is sometimes evenly 
disseminated through the feldspar, or may be arranged in bands, some- 
times wonderfully curved. These bands may be parallel in two adja- 
cent crystals separated by the mica of the cement, showing a formation 
in planes independent of the single albite crystals. Sometimes these 
bands occur only in the core of the crystal, and are bounded by a zone 
of clear albite, forming one crystal with the core. The outline of the 
banded core is then sharp and bounded by straight lines, producing an 
angular outline. In other cases the black material is irregularly dissem- 
inated through the core, but bounds it against the clear rim by a more 
or less continuous black line, which gives the core a rounded outline 
bounded by gentle curved sides, simulating quite closely the well known 
iron oxide bands which mark the limits of the original quartz grains in 
quartzites, enlarged by new silica. These are therefore apparenly en- 
largements of albite grains by new albite. Would it be possible instead 
to regard them as replacements of original feldspar grains by albite 
which grew beyond the limits of the original grain, did not entirely 
resorb the iron products, and sometimes affected their distribution by 
‘any pressure and movement which may have accompanied the chemical 
process by which the albite was formed ? 
HarvarpD UNIvERSITY, June, 1891. 
