PALEOZOIC TIME UPPER SILURIAN. 639 



iston. Thence they continue westward through Ontario with a thickness of 

 300 to 400 feet, and in eastern Ohio tliin out to 10 to 150 feet of reddish 

 shale (as found in deep borings) resting on Hudson or Utica shales. They 

 are not found in Michigan. To the southward, in New York, the formation 

 disappears beneath the later beds ; but it reappears on the west slope of the 

 Kittatinuy Mountains, and outcrops to the southwest in east central Penn- 

 sylvania, Virginia, and eastern Tennessee. 



The thickness of the Shawangunk grit is 500 feet in New York, and 700 

 to 800 feet in Pennsylvania ; and that of the Medina beds, in the latter state, 

 1800 feet. 



The beds thus give an idea of the seashore work of the period. They 

 also indicate the generally shallow depths of the Eastern Interior Sea, but 

 nothing as to the condition of the seas over the rest of the continent. 



The Clinton group has a wide distribution. Its beds occur in central and 

 ■western New York (the group taking its name from Clinton in Oneida 

 County), in Pennsylvania, Ohio, Indiana, Wisconsin, eastern Tennessee, 

 Kentucky, Alabama, and Georgia. The Cincinnati geanticline, which put 

 above the surface the Lower Silurian rocks, accounts for the absence of the 

 Clinton not only from part of Ohio, but also from western Kentucky and 

 Tennessee. In New York and Pennsylvania the rock is mainly a shaly sand- 

 stone and shale of rough, irregular aspect, with some intercalated limestone; 

 on the Niagara River it is about half limestone ; and in Ohio and farther 

 west almost wholly limestone. 



A peculiar feature of the formation is the occurrence in many regions of 

 one to three beds, 1 to 10 feet thick, of argillaceous red iron ore (hematite), 

 usually ooly tic, with the grains round or flattened. The grains are often con- 

 centric in structure, proving them to be true concretions, like those of an 

 ordinary oolyte, and of sea-margin origin. (C. H. Smyth, from observations 

 near Clinton, N.Y., and elsewhere in 1892.) 



These ore-beds accompany the Clinton formation from New York to Ala- 

 bama, through Pennsylvania, Virginia, eastern Kentucky, and Tennessee, 

 and also occur in Wisconsin. They are usually fossiliferous, and the ore is 

 sometimes called the " red fossil ore." The fossils are broken, and include 

 stems of Crinoids, Bryozoans in small fragments, and other species. The 

 beds were evidently made over tide-washed, salt-water flats, where tritura- 

 tion was gentle. They indicate a wonderful degree of uniformity in conti- 

 nental level over a wide area. 



Clinton fossils occur with those of the Niagara at some points along the 

 Atlantic border of Maine, from the boundary of New Brunswick to Penob- 

 scot Bay. They are found also in Nova Scotia, and on Anticosti several 

 hundred feet of limestone are referred to the Clinton. 



The Niagara formation is still more widely spread than the Clinton, 

 though far from continental in its distribution. In most regions it is a thick 

 limestone, but in New York and other parts of the Eastern Interior Sea, the 

 lower portion is shale, indicating a gradual transition in rock-making condi- 



