PHYSICAL AND FAUNAL EVOLUTION 241 
fissile shales, including an iron sandstone, and with Buthotrephis in 
the upper part. This is succeeded by 110 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 Lower 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 positions. 
Cross-bedding and irregular wedging-out of Jayers and a rapid thin- 
ning away of the entire mass, further suggest such an origin. There 
are no fossils in the ore, and it rests upen an uneven surface of the 
Upper Ordovicic, with a laver of highly polished clay pebbles marking 
its base. The interpretation of this formation that I am at present 
able to advance is that of a dune of calcareous pellets of concretionary 
or phytogenetic origin, similar to the odlite dunes of Great Salt 
