48 PROCEEDINGS OF THE NATIONAL MUSEUM. tod. 6G. 



The hornblende which, in the altered zone, takes the place of the 

 augite of the unaltered rock, is intensely pleochroic with X = pale 

 brown, Y = deep greenish brown, Z = deep blue green. Absorption 

 Y> Z> X. It is biaxial negative with 2V medium, Extinction Z A 

 c= 16° maximum. Sections perpendicular to the prismatic elonga- 

 tion show well developed prismatic hornblende cleavage. Toward 

 the border of the altered streak the hornblende grades into normal 

 augite of the pinkish-brown type and cores of the augite are sur- 

 rounded by the hornblende, lying in parallel position and extending 

 inward by replacement. 



One large grain of calcite was seen along the crack and this calcite 

 contained included tufts of fine fibrous colorless hornblende, grown 

 out from adjacent chlorite. Where chlorite abuts against a small 

 open cavity the tufts of byssolitic hornblende occur grown in 

 crystallographic continuity on the chlorite. 



Titanite occurs as idiomorphic crystals included in the chlorite and 

 also in the adjacent rock of the altered zone in all stages of replace- 

 ment of the skeletons of original magnetite. 



The original feldspar of the altered streak is completely filled with 

 close packed sericitic alteration product, so that it is not possible 

 to determine whether any replacement of the orginal plagioclase by 

 albite has taken place. There are later borders of clear albite grown 

 around cores of the original sericited feldspars, however, and where 

 these abut against the chlorite filhng, the outhnes of the broken 

 plagioclases have been completed by albite deposited from solution. 



It will be noted that the alteration here described is identical with 

 that along the diopside seams except in the nature of the ferromag- 

 nesian mineral which replaces the augite. 



Where, as shown in the illustration, this seam crosses the coarse 

 pegmatite streak, the crack continues shgirply across. Whereas in 

 the normal rock the alteration was confined to within some 3 mm. 

 of the crack, in the coarse pegmatitic rock the agents producing the 

 hornblendization of the augite, sericitization of the feldspar and 

 replacement of iron ore by titanite were able to effectively penetrate 

 farther and a completely altered large skeleton magnetite occurs 

 fully 2 cm. from the crack. 



One mineral was observed with the sericite as an alteration 

 product of the feldspar adjacent to the crack which could not be 

 identified. This was especially noted in an altered feldspar of a 

 patch of micropegmatite. The mineral occurs as grains and small 

 prisms which are colorless and have an index of refraction lower than 

 that of the hornblende. It was roughly estimated to be about 1.60. 

 It has strong birefringence and some sections give abnormal blue 

 interference colors. It is biaxial positive with 2V small. The dis- 



