BLACK HILLS GEOLOGY. 251 



invariable automorphism. They are penetrated by the aegirine 

 needles of the groundmass, as also are the quartz grains. 



Bisilicates are present in two distinct varieties which present 

 no essential difference from those in the phonolite family. A 

 more extended description of them will be found under the dis- 

 cussion of the latter rocks, but a brief description is inserted 

 here to render the account of the quartz-segirite-porphyries 

 complete. The two varieties are : 



^girine-augite in large automorphic crystals, and fine needles 

 of segirine, which penetrate all individuals of the groundmass 

 indiscriminately. The aegirine-augites are prevailingly perfect 

 in their crystal outline. 



c : c 7^"", 76°, and 75°. 



A very strong pleochroism is generally exhibited and is 

 greenish-brown, parallel toe. and deep bluish green parallel to a. 



Colorless, non-pleochroic cores of augite are frequently 

 observed. 



The aegirines of the groundmass exhibit in their pleochroism 

 a much deeper green, and are invariably parallel in their extinc- 

 tion. They exhibit both shredded extremities, and crystal ter- 

 minations. At times they are seen in bundles grouped radially 

 around some earlier formed large crystals of aegirine-augite, and 

 then seem usually to possess very perfect terminations. The 

 breadth sinks at times to a mere thread, so that the color of the 

 mineral can be distinguished only by the aid of the high power 

 objective. 



The quartz is confined entirely to the groundmass and differs 

 in no respect from that in the rock described from Lost Camp 

 creek. This rock shows in most respects a very close resem- 

 blance to the grorudites described by Broegger. We have, 

 however, again to call attention to the entire absence of horn- 

 blende and albite. 



Chemical Composition. — A partial analysis is given below. 

 No. I is quartz-aegirite-porphyry from Elk Mountain. No. II 

 is from Broegger, and is a grorudite from Kallerud.^ 



^Die Eruptivgesteine des Kristianiagebietes, p. 49. 



