is<)7.] DRAKE — THE GEOLOGY OF IXDTAX TERRITORY. 847 



the area the strata dip slightly to the west, while along the southern 

 border they are somewhat disturbed by gentle folds which usually 

 run a little south of west by north of east ; but many run about 

 northeast by southwest. The general result of the disturbance is to 

 make the strata dip with varying steepness to the south. 



Eureka Shale. 



This shale is black, argillaceous, bituminous and rather friable 

 and varies in thickness from one inch to forty or fifty feet. It 

 immediately overlies the Silurian beds and is apparently uncon- 

 formable with them since the shale lies on cherts and saccha- 

 roidal sandstones along Spavinaw creek and Illinois river, while 

 about twenty-five miles farther south it lies on higher strata com- 

 posed of one hundred and fifty feet or more of Silurian marble. 

 Similarly, in Arkansas, the shale lies on top of the St. Clair 

 marble and the Izard limestone along the southern border of the 

 Silurian area, while farther to the north it rests on cherts and sac- 

 charoidal sandstones which underlie the marbles and limestones. 



The shale is apparently conformable with the overlying beds, 

 since the limestone at the base of the Boone chert was found resting 

 on it at every place where it was seen. The shale may be seen at 

 nearly every locality where Silurian beds are exposed, and at a few 

 places where the erosion has not passed through the shale bed. 

 Quite often detritus from overlying rock beds covers and obscures 

 the outcrop of the shale, but by tracing the horizon a short distance 

 it is usually found. 



The Eureka shale has a wide distribution outside of the Indian 

 Territory. It is usually found in northwestern Arkansas wherever 

 the base of the Boone limestone is exposed. From its typical de- 

 velopment at Eureka springs. Ark., it was named by Dr. J. C. 

 Branner, the Eureka shale. ^ 



Dr. Branner says : " The Eureka shale is clearly the equivalent 

 of the Tennessee bed called by Safford the ' Black shale.' " - Saf- 

 ford says this shale also occurs in Virginia, Georgia and Alabama.^ 



^Ahh. Rept. Geol. Sur. Ark., Vol, iv, 1890, p. 345 ; " Phosphate Deposits of 

 Ark.," Tran. Am. Inst. Mm. Eng., 1896, xxxvi, 580-582. 



"^ Ann. Rept. Geol. Siir. Ark., Vol. iv, 1888, p. 26; "The Phosphate Deposits 

 of Arkansas," Trans. Am. Inst. Min. Eng., 1896, xxxvi, 582. 



3 Elementary Geol. of Tenn., by James M, Safford and J. B. Killebrew, Nash- 

 ville, 1885, p. 75. 



