GENERAL GEOLOGICAL CROSS-SECTION 271 



the similarly named formation of the Silurian of the New York section, 

 and can not be considered as having any legitimate claim to the title 

 which it usually bears, less confusion is likely to arise by continuing for 

 the present the old name than by introducing a more appropriate but 

 new term. 



The important point to be taken into consideration in the present 

 connection is the fact that the " Niagara " dolomite when met with is a 

 terrane about which there is never any doubt as to stratigraphical loca- 

 tion. It is a datum plane from which all higher geological sections may 

 be at once referred. 



LOWER PALEOZOIC FORMATIONS 



Of similar importance in the correlation of the Interval deposits are 

 the Ordovician strata of the region. Such formations as the " Hudson " 

 shales and the " Trenton "' limestone, for examples, are well developed 

 over broad areas. These terranes everywhere afford good checks in 

 paralleling vertical sections in localities more or less widely separated 

 from one another. 



THICKNESS OF THE INTERVAL DEPOSITS 



The space between the basal plain of the Carboniferous and the top 

 of the Silurian is not great when compared with the great thicknesses of 

 strata in other regions. In northern Iowa, where the maximum meas- 

 urement is greatest, the distance separating the two levels is about 450 

 feet. To the southward this figure gradually grows less and less until 

 in north Missouri it is not more than 150 feet, including the thickness 

 of the Hannibal shales and the Louisiana limestone. Around the western 

 flank of the Ozark dome thjs value is reduced to zero, but in northern 

 Arkansas it again expands to nearly 100 feet. 



On the eastern side of the Ozark uplift the Interval deposits are at no 

 point reduced below about 75 feet. 



Formations represented by the Interval 



iowa terranes 



The formations referred to the Devonian have received of late years 

 a number of new designations. The general section, however, may be 

 given as follows : 



General Devonian Section of loiva 



Feet 



Lime Creek shales 100 



Cedar Valley limestone 150 



Wapsipinicon limestone 150 



Independence shales 50 



