76 AMADEUS W. GRABAU 



fissile shales, including an iron sandstone, and with Buthotrephis in 

 the upper part. This is succeeded by no feet of calcareous fossilifer- 

 ous shales; and this by 230 feet of fossiliferous shales and limestones 

 with a Niagaran fauna. Above this follows 350 feet of red shales, 

 probably representing the Upper Salina, and separated by a hiatus 

 from the fossiliferous Niagaran shales. 



In eastern New York, at Swift's Creek, the type locality for the 

 Clinton, this formation is 226 feet thick and is followed by 5 feet of 

 Niagaran and then by the red shales of the Upper Salina. On the 

 Niagara River the Clinton shale with the two succeeding limestones 

 has a total thickness of 32 feet, followed by 68 feet of Rochester shale. 

 The total of the Niagaran, including the Guelph, is from 270 to 

 325 feet, as shown by borings. This is followed by Lower Salinan. 

 In the Rochester region the Clinton has a thickness of 80 feet, includ- 

 ing the Irondiquoit or upper limestone (17 feet), which Chadwick 

 refers to the Rochester. The eastward thinning of the Upper Nia- 

 garan beds indicates either that these beds were eroded before the 

 deposition of the red shales, probably during the Shawangunk epoch 

 (see beyond) ; or that the Rochester-Lockport of the West is in part 

 represented by Upper Clinton in the East. The Guelph element 

 may never have extended to the Clinton type region, which may have 

 been above water and so subject to erosion. 



The most typical section of the North American I>ower Siluric 

 or Niagaran is found in Wisconsin, where the series exceeds 700 

 feet in thickness and is wholly calcareous. At the base of the series, 

 however, in a few localities, as at Iron Ridge, occurs a remarkable 

 iron ore, composed of flat lentils of varying size and heaped together 

 in a mass strongly suggestive of dune history. This idea is borne out 

 by the position of these pellets, which are not laid flat, as would be 

 the case if they were deposited by water, but are placed in all poshions. 

 Cross-bedding and irregular wedging-out of layers and a rapid thin- 

 ning away of the entire mass, further suggest an eolian origin. There 

 are no fossils in the ore, and it rests upon an uneven surface of the 

 Upper Ordovicic, with a layer of highly polished clay pebbles marking 

 its base. The interpretation of this formation that I am at present 

 able to advance is that of dunes of calcareous pellets of concretion- 

 ary or phytogenetic origin, similar to the oolite dunes of Great Salt 



