630 W. H. TWENHOFEL AND F. T. THWAITES 



better stratified than are those immediately below. They are also 

 usually more or less dolomitic, and at or near the top there is quite 

 generally a hard quartzite-Hke sandstone to which the field name 

 of "clinkstone" was given. Up to the present these sandstones 

 have yielded no fossils other than worm borings which resemble 

 those found in the typical Madison sandstone. The thickness 

 varies from 2 to about 20 feet. 



If these sandstones are really of Madison age it is probable that 

 an erosion interval occurred between the deposition of the Jordan 

 and the Madison. The decisive evidence for such an interval has 

 not been found in these quadrangles, although suggested by the 

 considerable variation in the thickness of the Jordan. Such 

 variation in thickness, however, is readily explained through 

 inequality of deposition, which is a rather usual condition in sand- 

 stone deposits. Near Madison the Mendota dolomite is said by 

 Ulrich to hold a stratigraphic position between the Madison and 

 Jordan formations.' If this be correct and these sandstones are 

 really of Madison age, there is Httle doubt of the presence of a 

 disconformity. 



ORDOVICIAN SYSTEM 

 INTRODUCTION 



Only two formations of the Ordovician system are present 

 within the two quadrangles. These are the Oneota dolomite (in 

 the arrangement of the systems of Dr. Ulrich the Oneota and the 

 Madison are referred to his Ozarkian system) and the St. Peter 

 sandstone, each markedly different from the other in character, 

 origin, and distribution. The two formations are separated from 

 each other by a decided disconformity of marked relief. 



THE ONEOTA FORMATION 



The Oneota formation underlies the higher uplands and, except 

 for small outliers, is largely confined to the southern halves of the 

 two quadrangles. These upland areas have gentle outlines and 

 slope with slight inclination in a southwesterly direction. 



'E. O. Ulrich, "Correlations by Displacements of the Strandline, etc.," Bull. 

 Geol. Soc. Amer., XXVII (1916), 460. 



