.^H 



498 GEOLOGY OF THE BLACK HILLS. 



almost as if small portions of the crystal had been broken off by the action 

 of the once fluid fj^roundmass, which completely envelopes these fragments 

 while carrying them away. 



The hornblende crystals are very conspicuous as small yellowish-green 

 blades; they are sharply terminated and outlined, the sections in the differ- 

 ent planes affording long, slender, and small rhombic forms, respectively. 

 It is, of course, rather rare that any of these crystals, which lie in every 

 possible position, are cut so nearly at right angles to the vertical axis as to 

 afford a perfect rhombic section and to show the lines of cleavage, but it 

 was observed in a few cases with the careful use of an objective of very 

 high power. The crystals are also plainly dichroitic, changing with one 

 nicol from yellowish-green to a deep-green. A few large hornblendes show 

 this characteristic more plainly, but they are not so sharply terminated. 

 On the other hand, some are so minute as to be just discernible in great 

 quantities in the mass, sometimes heaped up or collected in bunches in a 

 re-entrant angle of a sanidin crystal, and filling also the interstices between 

 the larger prisms. The groundmass is colorless, but polarizes in tolerably 

 bright colors — red, blue and yellow — as if it were feldspathic In its ap- 

 pearance, it shows distinct signs of a former fluidity so marked and general 

 in all parts of the section that the wave-like structure is a striking feature. 

 These waves, so to speak, sometimes meet the opposing extremity of a 

 crystal and divide into two streams, one on each side, leaving at the point 

 of division quite a number of the hornblende crystals crowded together 

 and heaped up like logs in a river when an island has interrupted their 

 motion down the stream. Again, the once fluid mass fills all the fissures, 

 cavities and cracks in the feldspar crystals, the very small fissures contain- 

 ing onl}'- the microlites of hornblende, as if the larger blades had been pre- 

 vented from entering. The whole phenomenon is quite interesting. A 

 little magnetite is present, but in small quantities only. The rock contains 

 63.5G per cent, of silica, and 8.58 per cent, is soluble in hydrochloric acid. 



The sanidin-trachyte [124] from southeast of Terry Peak is of a gray 

 color, containing large crystals of sanidin and immerous small, dark horn- 

 blende crystals. It is also quite porous in structure, the holes resembling 

 pavitio.s which have been once filled by some crystallized mineral. Under 



