490 GEOLOGY OF THE BLACK HILLS. 



ered as always absent from the feldspars of trachytic rocks, but Zirkel men- 

 tions several cases as occurring' in the sanidin both of rhyolite and trachyte 

 in the rocks of the "Fortieth Parallel." Twin crystals accordi-ng to the 

 Carlsbad law are very frequent, even the minute crystals often showing 

 this peculiarity in polarized light. Broken crystals are also observed, which 

 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 space has been filled up again by the groundmass This is 

 well exemplified in one instance in the Black Hills trachytes, there being a 

 large sanidin crystal broken in two parts and the fragments displaced 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 distinguish 

 it from augite. Its sections are distinguished 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, 

 columnar 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 by being strongly dichroitic in the prismatic sections. The hexag- 

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



