840 THE GEOLOGY OF MINNESOTA. 



[Dioryte. 



paratively small amount of hornblende in ragged shapes, and of chlorite. The most 

 of the rock is composed of an interlocking network of finer gi-ains of rjitnrtz, calcite, 

 sericite, feldspar whose specific determination is impossible, and shreds of hornblende 

 and of chlorite, with occasional cubes of pyrite. These minerals are certainly all of 

 secondary generation, and are due to the stresses of pressure and of heat, with under- 

 ground moisture, to which the originals were subjected. It is not likely that this 

 rock was ever truly molten, but was a debris which has taken on a recrystallization, 

 the large feldspars being the only original minerals whose forms can now be 

 identified; and it is probable that they also were at first not albite. One section. 

 Age. "Intrusive"' amongst the Upper Keewatin. N. H. w. 



No. 2104. DIORYTE. 



Small island near the west shore of Long lake, N. W. % S. W. J^ sec. 29, T. 63-12 W. Apparently a condi- 

 tion of the prevalent greenstone. 



Ref. Annual Report, xxiv, page 42. 



Meg. Clean, gray, medium grained, composed essentially of white feldspar and 

 hornblende, not showing schistose structure, but having slight, irregular cloudings 

 of darker tint. 



Mic. The minerals are hornblende, feebly pleochroic, with sharp fibres that 

 extend beyond the average outlines of the grains and pierce the feldspar and quartz, 

 and also sometimes run in the same way into the fibrous portions of other horn- 

 blendes. These fibres are certainly of secondary growth and polarize less highly than 

 the body of the grain to which they belong. The color in the same grain may run 

 downward from blue, or the first " sensitive " violet, through all the colors to white, 

 the last being at the terminations of the fibres. In the same way secondary growths 

 of hornblende appear scattered throughout the grains themselves, as shown by the 

 white areas resembling the fibrous terminations, and occasionally the sharp fibres of 

 one hornblende pierce the older portions of another, the structure being somewhat 

 interrupted in its passage, but on emerging on the other side such fibres extinguish 

 in unison with the same fibres in the parent mass. It is apparent that such pene- 

 trating fibres made their way through the older hornblende at a time of general 

 metasomatic rearrangement, more or less complete. In their passage through the 

 body of the old hornblende they did not perfect their crystalline structure, for they 

 are marked by light streaks that do not wholly extinguish in an entire revolution. 

 There are also multitudes of little hornblendes more or less globular or club-shaped 

 scattered through the matrix. 



The feldspars, which are playioclase, are so darkened between crossed nicols that 

 they cannot be determined specifically. They are frequently angular and lie in a 

 fine matrix, much fresher, of the same materials. They are not porphyritic, but 

 imperfect or fragmentary, and they also show narrow rims of secondary growth. 



