BLACK HILLS GEOLOGY. 283 



The pleochroism is : 



Deep olive green ||c. 



Faint yellowish green || to c. 

 II to b. 

 Absorption c>b>a. 



The chlorite, which occurs as large masses, has resulted 

 from the decomposition of the hornblende, being prevailingly 

 pseudomorphic after that mineral. 



The groundmass is composed of irregularly bounded, lath- 

 shaped masses of striated feldspar, some undoubted orthoclase, 

 and an abundance of magnetite in grains and irregular masses. 

 Chlorite, calcite and secondary quartz are present as alteration 

 products. The following is a partial analysis of the rock from 

 the Rua mine : 



SiO^ 55.26 



AlA 17.67 



Fep.^ 5.39 



CaO 5.26 



MgO 3.21 



Kp not det. 



Na^O not det. 



loss 4.53 



H,0 .45 



Two other occurrences of the rock are worthy of note. One 

 is in the town of Terry in a thick sheet of considerable develop- 

 ment, which is also exposed on both sides of Fantail Gulch 

 along the railroad. The rock is here much finer grained than 

 that described, and is almost granular. The crystals of horn- 

 blende are much more widely disseminated through the rock, 

 although somewhat smaller, and in all cases altered to chlorite. 

 Biotite is less abundant. Garnet sometimes occurs ; otherwise 

 the rock shows no difference from the usual type. 



The other occurrence of diorite -porphyry is very important. 

 It is on the west of Spearfish Creek in the up-lift known as the 

 Needles (see p. 224). It is a highly porphyritic rock, with 

 even-tinted light-gray groundmass, in which are embedded white 

 phenocrysts of feldspar and abundant glittering blades of horn- 



