536 F. T. THWAITES 



Mayville,^ The bedding of the several formations varies from thin 

 and regular in the Byron formation to massive in parts of the 

 Waukesha and Racine formations. Indeed the separation of the 

 formations in drill records is very difficult if no data on thickness 

 of bedding are available. Chert of white and less commonly 

 blue color is found in all the formations; it is most abundant in 

 the basal Mayville and in the upper part of the Waukesha. The 

 thickness of the whole Niagaran series varies from 300 to 670 feet 

 increasing toward the northeast. 



ORDOVICIAN SYSTEM^ 



Medinan Series 

 " clinton" or ned a formation 



General. — The "Clinton" or Neda formation occurs only in 

 local lenses which outcrop in Dodge and Door counties, Wisconsin. 

 These rise as mounds from the even surface of the Richmond. 

 The Neda is found so rarely in deep wells that it requires no further 

 discussion than to state that the deposits are ooHtic hematite with 

 subordinate red shale layers and shale pebbles.^ The greatest 

 known thickness is 55 feet at Manitowoc, Wisconsin. 



RICHMOND GROUP 



Distribution. — The Richmond group ("Cincinnati" or "Hudson 

 River" formation of older reports) outcrops in a narrow belt 

 along the Niagara escarpment in eastern Wisconsin and north- 

 eastern Illinois. In southwestern Wisconsin and northwestern 

 Illinois there are many patches of Richmond beds, there called the 

 Maquoketa shale. 



Character. — The Richmond group in eastern Wisconsin consists 

 mainly of dark blue calcareous shale, for the greater part very 



^W. C. Alden, "The Quaternary Geology of Southeastern Wisconsin," U.S. 

 Geol. Survey Prof. Paper 106, (1918), pp. 89-90. F. T. Thwaites, "Recent Discoveries 

 of 'CHnton' Iron Ore in Eastern Wisconsin," U.S. Geol. Survey Bull. 540, (1913,) pp. 

 338-41. 



^ The Richmond group is placed in the Silurian by Ulrich, but all older writers 

 and the majority of geologists at the present time place it in the Ordovician. 



3 The conclusions expressed by the writer in 19 13 as to the feasibility of mining 

 this ore under heavy cover need revision. There is now little question that the red 

 beds near Kenosha are not iron ore, and that a fault occurs near Manitowoc; this 

 would cause great difficulty with water at the latter locality. 



