574 SAMUEL CALVIN 



The Maquoketa shales were so named by White in his report on 

 the Geology of Iowa, pubHshed in 1870, The beds are, in part 

 only, the equivalent of the Cincinnati shales of Meek and Worthen, 

 of the Hudson River shales of the New York geologists. 



The Niagara limestone. — Lithologically, the Niagara series of 

 Iowa is wholly unlike that of New York. There are no sandstones, 

 no shales, practically no unaltered limestones. In the mid-west 

 the Silurian is represented by a great body of dolomite in which 

 there is more or less commingling of the Clinton and Niagara faunas 

 of the region farther east. In some cases a number of life zones 

 may be recognized. Syringopora tenella characterizes one of these; 

 Pentamerus oblongus, another; another has Caryocrinus, Eucalyp- 

 hcrinus, and related forms as diagnostic types; and others, like 

 that carrying Dinoholus Conradi, are marked by still different species. 

 But these zones are not well set off one from the other, and in many 

 localities there is more or less of interminghng of forms from adjacent 

 zones. The lower part of the Niagara limestone, including the 

 zones between the base of the formation and the top of the Pentam- 

 erus-bearing beds, is quite distinct from the upper portion which 

 includes what have been called the Le Claire and the Anamosa 

 limestones. In the earlier volumes of the current series of Iowa 

 Reports the lower phase was designated the Delaware stage, and 

 the upper has been called by Norton the Gower stage. The term 

 "Delaware," however, as the name of a geological unit, was used 

 by Orton for a phase of the Ohio Devonian as early as 1878, and 

 it is proposed in the Winneskiek County report, now in press, to use 

 "Hopkinton" in place of the preoccupied term "Delaware" for 

 the lower phase of the western Niagara. All the characteristics 

 of this stage are well displayed in the quarries and ravines within 

 a radius of two or three miles around Hopkinton in Delaware County, 

 Iowa. 



The Devonian system. — The Devonian is represented in Iowa by 

 an assemblage of sediments carrying characteristic Devonian faunas. 

 It is not possible, however, definitely to correlate any part of the 

 western Devonian with any part of the sediments referred to the 

 same system in New York. There is certainly nothing west of the 

 Mississippi which can be said to represent the Helderberg or Oris- 



