870 GRASSES OF IOWA. 



Cambrian. The Cambrian sandstones outcrop along the Mississippi 

 river and its tributaries in Allamakee and Clayton counties, and occur in 

 the valley of the Oneota or Upper Iowa river as far west as Winneshiek 

 count}-. The maximum thickness of the formation, which is estimated by 

 Norton at about 1,500 feet 1 , is nowhere exposed, only the upper part 

 being seen in Iowa. The great bulk of the Cambrian is made up of the 

 Basal sandstone which is covered by the Lawrence shales and limestone, 

 and this in turn by the Jordan sandstone. 



Ordovician. Above the latter is the Ordovician or Lower Silurian, 

 which occupies the surface between the Cambrian outcrops and the Niag- 

 ara escarpment, running from northeastern Howard county to Clinton. 

 The Ordovician here consists of four members with minor sub-divisions, 

 the whole having a maximum thickness, as estimated by Norton, about 

 equal to the Cambrian. The lower member is the Oneota dolomite with 

 its upper and lower divisions separated by the New Richmond standstone. 

 It is this rock which mainly forms the picturesque cliffs of Allamakee 

 county and which is often known in geological literature as the Lower 

 Magnesian. Above it is the Saint Peter, a sandstone of remarkable pu- 

 rity and uniformity in character and thickness. The Saint Peter is cov- 

 ered by the Galena-Trenton series, really one formation of which the 

 lower portion is undolomitized, and is known as the Trenton, while a 

 varying portion is dolomitized from the top downward and has been 

 named the Galena. The rocks of this formation form the cliffs around 

 Dubuque, and are the source of the lead and zinc ores of the state. 

 Above these cliffs, and stretching by easy slopes up to the base of the 

 Niagara escarpment, are the Maquoketa or Hudson river shales. 



Niagara. The Niagara or Silurian, is divisible into several minor 

 formations 1 but for ordinal}- purposes may be considered as one great 

 idolomitic bed stretching across the northeastern portion of the state. 

 Where not too heavily covered by the drift it yields a very rugged topog- 

 raphy. Usually, however, the great drift plain sets in at the outer edge 

 of the formation so that there is little by which to topographically dis- 

 tinguish its area of outcrop. 



Devonian. Southwest of the Silurian is the Devonian which is very 

 largely made up of non-dolomitized limestones. In its upper portion it 

 contains an important shale bed, the Lime Creek, noted for its well pre- 

 served fossil remains. The Devonian also includes an important breccia 

 which indicates that the conditions were not wholly undisturbed in this 

 middle region in Devonian time. There is in addition an unconformable 

 member as yet known only in Johnson and Muscatine counties, and 



♦Iowa Geol. Surv. , vol. 6: pi. 6. 

 •IowaGeol. Surv., 5:48-60. 



