850 DRAKE — THE GEOLOGY OF IXDIAN TERRITORY. [Sept. 3, 



Slratigraphy. — Most of the formation is chert, and the limestone 

 strata are usually confined to the base and top of it. These 

 two limestone horizons are usually five to twenty feet thick each. 

 The limestone interstratified with the chert is in thin layers, sel- 

 dom more than two or three feet in thickness, and is of rather rare 

 occurrence. 



Characteristics of the Chert. — The chert varies in color from 

 white to gray; light gray or white is the prevailing color, and is 

 especially characteristic of the rock that has been exposed to 

 weathering agencies, or is dry. Weathered pieces, however, are 

 often of a brownish color. The rock is almost a uniform mixture 

 of lime and silica, but the proportion of lime and silica varies 

 somewhat at different horizons or even along the same stratum. In 

 weathering it breaks into rather small sharp angular pieces. The 

 strata are usually thin and present a slight wavy and much fractured 

 appearance. These features are doubtless due to the weather- 

 ing of rock of an uneven composition. These irregularities and 

 characteristics run through the whole chert bed throughout the 

 entire area. 



Limestones. — The limestone at the base of the chert bed corre- 

 sponds to the St. Joe marble of the Geological Survey of Arkan- 

 sas '^ it varies from an inch to about thirty feet in thickness ; it 

 is of rather uniform texture, quite crystalline, of a rather dark-gray 

 color, and lies in strata from a few inches to four or five feet in 

 thickness. This bed is rarely missing where the base of the Boone 

 chert was seen. On Spavinaw creek and Illinois river it is from fif- 

 teen to twenty feet thick. On Dry creek, one and a half miles 

 northwest of Marble, it is about two feet thick, crystalline, gray in 

 color, and contains greenish specks scattered through it. 



The limestone that occurs very sparingly through the chert is 

 rarely more than two or three feet thick. It is usually of uniform 

 texture, tough, and of a gray color. 



The limestone at the top of the chert is from fifteen to forty feet 

 in thickness. It is massive, tough, gray and crystalline, but in the 

 northwestern part of the area it is arenaceous and somewhat flaggy. 

 This limestone forms the base of a great many small hills along the 

 border of the chert area. It is often marked by glades due to thin 

 soil. Along the west bank of Spring creek, near the south line 



1 Ann. Rept. Geol. Siirv. Ark., 1890, Vol. iv, p. 253. 



