BLACK HILLS GEOLOGY. 223 



(i) Shales, 15 feet. 



(2) Quartz-^'Egirite-Porphyry, 160 feet.. 



(3) Shales, 54 feet. 



No. 2. was examined and is described in detail on a later page 

 As compared with the rock from the summit of the peak, it dif- 

 fers only in that it contains quartz as phenocrysts and has a 

 much finer ground mass, also in the fact that it contains few 

 large aegirites, but in their place a greater quantity of fine 

 needles of the same mineral. Hence I would take it to be a 

 peripheral phase of the same massif of which the rock on the 

 summit is the more slowly cooled representative. 



To sum up these data : 



The crest of Terry Peak is a mass of igneous rock, with the 

 strata on the northwest overlying it within 200 feet of the 

 summit, and dipping away at an angle of 20 degrees. The 

 thickness and extent of this mass seems to increase slightly to 

 the southeast. Sheets of similar rock occur in the Snowstorm 

 shaft, at the head of Nevada gulch, and a sheet of rock which 

 may prove to be of the same character is exposed at the head of 

 Raspberry gulch, to the southeast. Between the Snowstorm 

 shaft and the summit of the mountain great thicknesses of 

 horizontal strata are exposed. Horizontal strata are also ex- 

 posed in Ruby basin, to the southeast. From these facts we 

 are in a position to interpret the geological character of the 

 mountain. The mass of Terry Peak is probably composed of 

 a series of sills of igneous rock, varying in thickness and sep- 

 arated from one another by partings of Cambrian shale and 

 sandstone. Whether or not these were all derived from the 

 same conduit or series of conduits below it is impossible to say, 

 but such is probably the case. The present topographic sum- 

 mit of the mountain was not then a geologic center of disturb- 

 ance, but rather one of the thinner sheets intruded from the 

 southeast. 



That the capping sill was of great lateral extent seems to 

 follow from the almost granitic character of the ground-mass, 

 from the seemingly great size of the rock mass to the southeast, 



