72 



GEOLOGY OF THE BLACK HILLS. 



nacles, which near by resemble organ pipes and in the distance saw teeth, 

 and which contribute greatly to the picturesqueness of the scenery. They 

 undoubtedly owe their formation to a more readily decomposed material 

 which has been removed from between their surfaces, but they can hardly 

 be due to a true stratification in the granite itself. The peculiar pinnacled 

 topography to which they give rise is confined to a limited area extending 

 about three miles southwest from Harney Peak and is not generally observ- 

 able. The saw teeth and the weathering planes are illustrated by Figures 

 5 and 6, which represent scenery photographed near Harney Peak. 



In the summit of Harney Peak a small mass of mica schist was 

 observed cemented in the feldspathic rock, with its stratification nearly 



horizontal. It appears to be an entrapped piece of 

 schist enfolded by the granite when the latter was 

 in a plastic state. Mr. Jenney observed near Har- 

 ney Peak a huge fragment of schistose rock sim- 



>< ~i < \ i -/'' v L<.-i 





hJ^< ilarly entrapped in a mass of granite. This is 





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Fig. 7. — Plan and Section of 

 Schist included in Granite. 



represented in the annexed cut. The mass is about 

 100 by 75 feet in size and lies with its stratification 

 ^Fcffirh. vertical. Between the strata of schist is a seam of 

 the ferruginous quartz so common in that associa- 

 tion, and by weathering the rocks have been cut 

 away in the manner shown in the cross-section. 



To study with facility the relation of the granite 

 to the schitose rocks, we must leave the main granite 

 area and observe those outliers or ridges which run 

 among the schists and have no immediate connection with the central 

 mass. In them the character of the granite is similar in all respects 

 to that of Harney Peak, and we may safely assume that the main range 

 bears the same relation to the great schist area that we find them to bear to 

 the schists about them. That relation is in every case the same; they all 

 run parallel with the stratification, and are perfectly distinct and separate 

 from the schistose rocks. There is always an absolute and positive line of 

 demarkation between the two ; the transition is sudden, and in no case was 

 there observed any gradation of one rock into the other, though such facts 



