396 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



talliue. The hornblende occurs in long- prisms 3 to 10 mm. in length, which 

 lie close together, and inclose, partially surround, and abut against smaller 

 angular grains of plagioclase. The plagioclase is quite unstrained and is 

 usually fresh and clear, and entirely without crystal boundaries. Brown 

 biotite is universally present in small amount, in long plates parallel with 

 the foliation. It does not seem to be an alteration product from the horn- 

 blende. Quartz is the least abundant constituent. It is crowded with fluid 

 cavities and needles of rutile, and often incloses minute crystals of horn- 

 blende. The plagioclase, from its high extinction angles and alteration 

 products, is evidently basic. A little inagnetite is present, but titanite has 

 not been observed. 



The structural features are well brought out in thin section In the 

 linear-parallel type the hornblendes all lie with their crj^stallographic axes 

 parallel to a line. A thin section parallel to the foliation cuts essentially all 

 in the zone of the prism or near it; one across the foliation gives only sec- 

 tions across the prism. The grains of plagioclase are generally elongated 

 without strain. Their outlines are most irregular and quite independent of 

 the twinning lamellae. Their general appearance is that which would be 

 presented if numerous crushed contiguous grains had united by some proc- 

 ess of annealing or absorption to form the new individuals. In the plane- 

 parallel type the only difference is that the hornblende prisms have grown 

 parallel to a plane, in which, however, they may have any orientation An 

 indistinct banding is also often observable in this type, caused by a partial 

 grouping of the light and dark constituents in parallel layers. The order 

 of crystallization seems to have been plagioclase first, but nearly contempo- 

 raneous with the hornblende and biotite, and the quartz last. 



The amphibolites occur in comparatively narrow bands of indefinite 

 length in the granites and gneisses. The width usually does not exceed 8 

 to 10 feet, and their dip is always at high angles. The boundaries are 

 invariably sharp, and frequently cut the foliation of the amphibolite within 

 and of the gneisses without somewhat obliquely. There is a general uni- 

 formity of grain throughout the width; the wider bands are not coarser 

 than the nai-rower. 



