MUSEUM OF COMPARATIVE ZOOLOGY. 183 



Both microcline and probably albite have also formed independently 

 of any pre-existing nucleus, as far as their outline givea njy 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 aft'ected their distribution by 

 any pressure and movement which may have accompanied the chemical 

 process by which the albite was formed? 



Harvard Universitt, June, 1891. 



