THE TOM AH AND SPARTA QUADRANGLES 631 



The Oneota is quite commonly considered the lower member of 

 the Prairie du Chien formation. In the older nomenclature it 

 constituted one of the members of the Lower Magnesian group. 



Relations to underlying strata. — There is essential parallelism 

 between the beds of the Oneota and underlying strata, but it is 

 believed that the latter were eroded before the deposition of the 

 former. The basal bed of the Oneota is quite commonly a con- 

 glomerate, and mud cracking is not rare in the associated clayey 

 beds. The quartzite-like beds — the "clinkstones" — in the upper 

 portion of the Madison (?) may be a case of pre-Oneota case- 

 hardening resulting from exposure before Oneota deposition. Near 

 Madison bowlders of a quite similar quartzite are present in the 

 basal Oneota, suggesting that the foregoing is a possibility. The 

 thickness of the underlying sandstones varies from place to place, 

 and the upper surface of these sandstones suggests erosion before 

 the deposition of the Oneota. 



Characteristics. — The Oneota formation consists of firm and 

 compact dolomites in regular beds. Colors vary from gray to 

 drab. Some of the lower beds are oolitic. The basal beds are 

 somewhat sandy, and a conglomerate of dolomite and sandstone 

 pebbles in a sandy dolomite matrix is quite commonly present at 

 the base. This conglomerate and some of the immediately over- 

 lying sandstone beds contain flakes of green shale. The sandy 

 beds persist through about a dozen feet of strata. Quite commonly 

 the sandy beds are so firmly cemented as to form quartzite. A few 

 feet above the base there is a rather persistent bed of fine-grained 

 white sandstone which locally is firmly cemented to form a white 

 quartzite. Much gray, blue, and yellow chert is present through- 

 out, and some beds contain vast quantities. The chert of the 

 oolitic layers is also oolitic. The dolomite contains many small 

 cavities, some of which are lined with white, yellow, and amethys- 

 tine quartz. 



The maximum thickness of the Oneota in these areas is not 

 believed to exceed 170 feet. 



The most common fossils of the Oneota formation are irregular 

 masses of Cryptozoa. These are abundantly present as isolated 

 individuals and more rarely as reeflike masses, one of the latter 



