Crosby. 1 500 [March 7, 



spar in slender prisms, forming graphic granite. The mica ranges 

 from silvery white to dark brown in color. It is always highly 

 crystallized, and well defined hexagonal crystals two inches in di- 

 ameter are very common ; while mica plates six inches or more 

 across have been found. It forms only about 5 per cent of the 

 granite and is usually found in bunches or segregations. Besides 

 these three minerals, the only one found in abundance is black 

 tourmaline. It sometimes forms 3 or 4 per cent of the granite and 

 is usually highly crystalline. Crystals from three to eight*inches 

 in diameter and nearly a foot in length are not very exceptional." 



Many of the granite masses exhibit more or less distinctly the 

 banding and the comb structure so characteristic of true segre- 

 gation veins. In some of the smaller veins, especially, the prisms 

 of tourmaline are arranged with great regularity at right angles 

 to the walls and shoot through successive layers of quartz and 

 feldspar from the walls to the centre of the vein. Besides tour- 

 maline, the granite veins contain columbite and cassiterite ; and 

 the numerous tin mines which have been opened in the granitic 

 rocks of the Harney Range have made the determination of the 

 composition, structure and origin of the so-called granite questions 

 of great economic as well as scientific interest. The miners very 

 generally call the gangue of the tin ore greisen, although it con- 

 sists but rarely of quartz and mica alone. At the celebrated Etta 

 mine the cassiterite is found chiefly in a crystalline aggregate of 

 greenish white muscovite and albite (clevelandite) containing but 

 little quartz ; yet, on account of carrying the tin ore, this matrix 

 has been called albitic greisen. This vein, which is several hun- 

 dred feet wide and has been traced for a considerable distance, 

 affords, in its truly wonderful mineral developments, a complete 

 refutation of the view that these masses are eruptive. The rela- 

 tively finely crystalline albite-muscovite gangue of the cassiterite 

 is distributed irregularly among extremely coarse crystallizations 

 of albite, orthoclase and quartz, and gigantic crystals of spodu- 

 mene. Many of the spodumenes range from five or ten to thirty 

 and even forty feet in length, and from six inches to three feet in 

 breadth, with very perfect terminations. 



My own observations and Newton's description alike force me 

 to the conclusion that true eruptive granite is probably entirely 

 wanting in the Black Hills. The facts already cited make it more 

 than probable that this prodigious development of endogenous gran- 



