178 W. S. Bayley — Actinolite Magnetite Sc 



in the amphibole. The smaller grains, with the exception of 

 the tiny dust particles scattered through the amphibole, all 

 show traces of octahedral or cubic faces. The larger grains 

 are very irregular in outline, but their irregularity is due rather 

 to the agglutination of tiny grains with crystallographic con- 

 tours than to the accumulation of masses with accidental 

 boundaries. Traces of octahedral faces may be seen in nearly 

 all of the large magnetite masses, and in some cases these 

 latter appear to be sections of well developed crystals. 



The amphibole is in large colorless plates that interlock by 

 irregular sutures, or in sheaf-like bundles of tiny fibers. In 

 the former case the mineral includes magnetite so thickly that 

 in the hand-specimen the rock appears finely granular ; but it 

 reflects light evenly from large areas, and these upon close 

 inspection are learned to be the cleavage faces of the amphi- 

 bole. In cross section the plates sometimes present well 

 marked crystallographic outlines in which the prismatic planes 

 of hornblende are plainly distinguishable, and these sections 

 are crossed by two cleavages cutting each other at the horn- 

 blende angle of 124°. In other sections only a single cleavage 

 is apparent as a series of parallel lines against which the ex- 

 tinction is about 14°. The double refraction of the mineral is 

 fairly strong and is negative, and the plane of its optical axes 

 is parallel to the single cleavage. In two of the sections 

 studied many of the plates are crossed by poly synthetic twin- 

 ning lamellae as in the case of the griinerite described by Lane 

 and Sharpless. 



The fibers in the sheaf-like bundles are colorless like the 

 material of the plates or are very light green. Their extinc- 

 tion is sometimes parallel to their long axes and sometimes is 

 slightly inclined to them. Their double refraction is some- 

 what stronger than that of the plates and their polarization 

 colors are usually very brilliant. Their axes of least elasticity 

 are nearly parallel to the longitudinal directions of the needles 

 which are therefore negative, if the orientation of their optical 

 axes is as it is in normal hornblende. Two cleavages are dis- 

 cernible, one parallel to the long directions of the fibers and 

 one (a parting) transverse to them. Cross sections of the 

 bundles were not seen. 



The properties of the plates and of the fibers sufficiently 

 characterize both as a tremolitic or actinolitic hornblende or as 

 griinerite. 



A second amphibole intergrown with the colorless variety 

 differs from this latter only in having a greenish tinge. It 

 may also be slightly more strongly doubly refracting, but 

 otherwise it is similar in all respects to the colorless mineral. 



