490 



(lEOrOGY OF THE BLACK HILLS. 



ere<l as alwa\ 8 absent iioin the feldspars of tracliytic rocks, Ijut Zirkel men- 

 tions several cases as occurring in tlie sanidin both of ihyolite and trachyte 

 ill the rocks of the "Fortieth Parallel." Twin crystals according to the 

 Carlsbad law arc vci)- frequent, even the minute crystals often showing 

 this ])eculiarit}' in polarized liglit. Broken crystals are also observed, whicli 

 give rise to the explanation that the magma while soft had moved after 

 the crystals were formed or separated from the groundmass. The fragments 

 lie near each other and have the same rough, jagged line of fracture, while 

 the intervening sj)ace has been filled up again b}' the groundmass This is 

 well exemj)lified in one instance in the Black Hills trachytes, there being a 

 large sanidin crystal broken in two parts and the fragments displace<l by 

 another sanidin mass (Plate II, Fig. 2). 



Plagioclase is very often present with the sanidin, sometimes in large 

 crystals, finely banded, and when subordinate in quite small crystals the 

 bands are distinct enough for recognition under the microscope. 



Hornblende occurs in trachyte quite frequently as large crystals, which 

 are green or brown in color. They often have inclusions of magnetite and 

 a border or rim of magnetite grains similar to that of biotite. Glass par- 

 ticles are also often included by the large crystals. The lines of cleavage 

 and the dichroism can be observed in most cases, which serve to distingui.sh 

 it from auffite. Its sections are disting-uished from biotite under the micro- 

 scope by the cleavage lines, as well as^ by the different position of the optic 

 axes. With hornblende the sections parallel to the base are the most 

 strongly dichroitic, while those parallel to the prism are not at all or very 

 slightly so. The contrary is the case with biotite. The hornblende prisms, 

 colunmar crystals, and microlites, often abundant in the groundmass, are 

 generally green and vary greatly in size in the same rock from crystals to 

 very minute needles, as in [123]. The terminations are also different in 

 character, some being sharp with distinct crystal faces, while other crystals 

 have rough, ragged, or forked extremities. In some trachytes, there is but 

 little hornblende present. 



The brown crystals of biotite are often met with, and are easily recog- 

 nized b}' being strongly dichroitic in the prismatic sections. The hexag- 

 onal, basal sections jiave under the microscope an olive-brown color, are 



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