52 UNIVERSITY OF MISSOURI STUDIES 



well-rounded grains of quartz, is fine to coarse-grained, and usu- 

 ally white or yellow in color with some red phases. It exhibits 

 ripple-marks and cross-bedding. 



The sandy phases weather to a sandy soil and, as a rule, 

 form ledges because they weather more slowly than the dolomite 

 on each side of them. The thin-bedded dolomite weathers fast- 

 est, but because of the large amount of chert the entire series 

 weathers very irregularly. The result is a soil which is always 

 some shade of red, in which there is more or less chert, the 

 amount depending largely on the gradient of the surface on 

 which it is accumulating. Ball and Smith state that in Miller 

 County the Gasconade soils are thin. The upper portion of the for- 

 mation is relatively free from chert, hence the soils resulting 

 from its decay are excellent for agricultural purposes. In places 

 they are deep without containing any chert, as on the east branch 

 of Gravois Creek. 



There seems to be an unconformity at the base of the Gas- 

 conade in Miller County ; but Marbut suggests that this apparent 

 unconformity may be due to the solution of the surface of the 

 Proctor dolomite at its contact with the Gunter sandstone. There 

 does not seem to be an unconformity between the Gasconade and 

 the Roubidoux formations in this district. 



The Roubidoux formation. — The Roubidoux formation con- 

 sists of a very complex series of dolomite, cherty dolomite, chert, 

 and sandstone beds. Most of the beds are not persistent. Their 

 total thickness ranges from 70 to 160 feet. 



The dolomite ranges from fine to coarse-grained, but on 

 the whole it is fine-grained. Marbut notes a change in the tex- 

 ture from the rather coarse-grained dolomite in the lower part 

 of the formation to the very fine-grained Cotton Rock at its top. 

 As a rule, the lower portion is gray, of some shade, while the 

 Cotton Rock (local name for the upper part of the Jefferson 

 City) is yellowish to buff. The chert is distributed as in the 

 Gasconade formation except that it is more abundant and may 

 be in beds up to 30 feet in thickness. There is far more oolitic 

 and brecciated chert than in the Gasconade formation. The 

 chert ranges from dense white, gray, or black chert (flint) to a 

 more or less porous material. Cellular and honey-combed masses 



