INTRUSIVE SHEETS IN GALLATIN MOUNTAINS. 79 



spar was produced by dynamic forces acting on the whole rock. The 

 nearly parallel position of the cracks in all feldspars in one rock section 

 indicates that it was due to such action. The total absence of cracking in 

 the hornblende and the very fresh condition of this mineral are certainly 

 •remarkable (182). In one instance, where a large hornblende is adjacent 

 to a feldspar, both are cracked in the same direction, but the hornblende 

 exhibits no alteration along the cracks. The alteration within the feldspar 

 appears to be confined entirely to its crystal, and to depend upon the 

 feldspar substance, and not to be in the nature of an infiltration which 

 might have lodged within the cracks in the hornblende. At the margin of 

 the rock body the color is dark and the phenocrysts are very small and in 

 greater numbers (184). The groundmass is microcryptocrystalline, with 

 indistinct poikilitic patches. In other parts of the body the groundmass 

 has a microgranular structure with minute idiomorphic quartzes. This is 

 in contact with one of the segregations (188), which consists of an aggre- 

 gate of rectangular feldspars, for the most part in polysynthetic twins with 

 low extinction angles, and of larger hornblendes, with a small amount of 

 interstitial cement of feldspar and quartz grains. It contains long, slender 

 needles of apatite, partially altered magnetite, and some chlorite in flakes, 

 which suggest former biotite. The hornblende is precisely the same as 

 that in the surrounding rock, and the segregation is plainly a local modifi- 

 cation of the magma of the rock. One segregation consists wholly of 

 brownish-green hornblende. Another segregation consists of similar horn- 

 blende crystals crowded together, with rather large feldspars, and no fine- 

 grained cement. There is some quartz, calcite, and chlorite. The feldspars 

 are more or less altered, while the hornblende is perfectly fresh. The 

 banded segregations with gneissoid appearance consist of plagioclase and 

 hornblende, with biotite in places, considerable magnetite and green spinel, 

 besides chlorite as an alteration of biotite. There is little or no quartz. 

 The banded structure is due to the arrangement of magnetite, spinel, and 

 biotite in streaks or layers, and to the crystallization of part of the feld- 

 spar in small crystals and grains in layers. While the dark-colored miner- 

 als exhibit a pronounced parallel arrangement in places, the crystals of 

 feldspar lie in all possible positions, and share in the banded character only 

 by being in small grains or in large crystals. The hornblendes and some 

 biotites are poikilitic, inclosing small rounded crystals of plagioclase. The 



