1^8 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



development of the prismatic cleavages. The pleochroism is very- 

 strong with A, yellow, B, yellowish-green, and C, bluish-green. The 

 absorption scheme is the usual C> B > A. Very few basal sections 

 were observed, but when noted, the rather rarely developed macro- 

 pinacoidal cleavage (100) was distinctly seen. An occasional twin 

 was noted, and with the exception of an occasional apatite, a frag- 

 ment of feldspar, and perhaps a zircon or two, the hornblende con- 

 tains no included minerals. The feldspar, the light colored mineral 

 filling the interstices between the hornblende, has undergone consid- 

 erable change. In polarized light it appears rather clouded, though 

 not at all commensurate with the amount of change which it has 

 undergone. The true extent of this change may be seen by diminish- 

 ing the illumination ; then it appears that alteration is quite general, 

 that as a rule it follows the cleavage cracks or composition planes of 

 the twins, and that, as a result, a secondary mineral has been produced 

 with brilliant interference tints, probably sericite. To a very small 

 extent calcite is present in spots, as proved by faint effervescence 

 when the specimen is touched 'Cvith acid. Twinning of the plagio- 

 clase, according to the albite law, was noted, though it is not general, 

 and the high symmetrical extinctions indicate, together with those 

 on the pinacoids, that the feldspar is a labradorite of a composition 

 approximating AbiAn^ to AbgAug. It is evident that the feldspar 

 has accommodated itself to the space remaining after the crystal- 

 lization of the ferromagnesian constituent, and in the complete 

 absence of quartz was the last constituent to crystallize. 



Additional to the feldspar and hornblende occurs magnetite, most 

 commonly situated on the peripheral portions of, or enclosed by, the 

 hornblende. Usually it is surrounded by a colorless or light-green 

 rim, appearing as though formed at the expense of the amphibole. 

 An occasional red scale of hematite was also observed, associated 

 with the magnetite. Pyrite, already noted among the megascopic 

 constituents, is readily diagnosed in incident light. Tabulated in the 

 order of their abundance, there are present : 



i Hornblende. 

 _ , , . ( Sericite. 

 Labradorite i r- 1 •.. 

 ( Calcite. 



' Magnetite. 



Pyrite. 



. . . , Hematite. 



As Accessories < „. 



Zircon. 



Apatite. 



I, Garnet. 



