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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 57. 



Fig. 3.— Showing rela- 

 tion OF Dike (A) to vein 

 (B) on 2,000 foot levkl 

 OF Standard-Mammoth 

 Mine. 



Other coarse spessartites. — Certain dikes which accompany the 

 Osbum fault throughout a large part of its length closely resemble 

 these coarse spessartites, but they are enormously decomposed. 



Spessartites^ Standard Dike. — One of the most interesting rocks 

 studied vvas that which intersects the Standard vein in the Greenhill- 

 Cleveland Mine, near the center of the Standard-Mammoth ore shoot. 

 Specimens were collected from this dike on the 1,200-foot and 2,000- 

 foot levels of the mine. On the 2,000- foot level the dike cuts squarely 

 across the ore body, bulging slightly in the vein. 

 On the 1,200-foot level the dike is deflected 

 along the vein for about 20 feet. These rela- 

 tions are shown in figures 3 and 4. This dike 

 coincides in dip with the pitch of the ore shoot 

 and it can be traced upward to the No. 5 tunnel, 

 2,500 feet vertically, above the 2,000-foot level. 

 In the hand specimen this is a dark-gray equi- 

 granular fine-grained rock, showing minute 

 needles of hornblende and white grains of feld- 

 spar under a good lens. That from the deep 

 levels of the mine is quite fresh, but material from the No. 5 tunnel 

 is altered to a friable sandy brownish-green aggregate. The fresh 

 rock is cut by numerous joints which divide it into sharp-edged 

 rhombic or tetrahedral blocks. 



Under the microscope the rock is seen to be composed of prisms of 

 pleochroic greenish-brown hornblende with accessory and not very 

 abundant iron ore in small scattered grains, occasional small apa- 

 tites, and minute shreds of biotite in a beautifully crystallized 

 trachytic groundmass composed of twinned laths of plagioclase. In 

 the rock from the 1,200-foot level there 

 also occur occasional large phenocrysts, 

 which were apparently originally olivine 

 but which are now entirely altered to talc. 

 These are commonly surrounded by a rim 

 of hornblende prisms. They do not occur 

 in the rock from the 2,000-foot level. The 

 hornblende, which is well crj^stallized. is 

 intensely pleochroic in tones of deep brownish-green and pale green- 

 ish-brown. It is frequently twinned parallel to (100) . The extinction 

 angle measured from the twinning plane is around 12 degrees. This 

 is near basaltic hornblende grading toward common hornblende. The 

 plagioclase shows broad twinning lamellae and ranges in composition 

 from anorthite, AbpAn^j at the center to labradorite, AbiAn^, at the 

 peripheries. There is a gradual shading from center to border with 

 no distinct zones. The rock from the 1,200-foot level may well be 

 called an olivine-spessartite in view of the original occurrence of 

 olivine. The rock from the 2,000-foot level is a typical spessartite. 



Fig. 4.— Showing relation of dike 

 (A) to vein (B) on 1,200 foot level, 

 of Standaud-Mammoth Mine. 



