50 NIAGARA GROUP. 



of it, and to represent a border-laud and shallow water deposit, that extended 

 only a short distance from the primitive source of its materials. 



102. In Western New York the dividing line between the Medina and 

 Clinton is sharply defined, and the materials of which each are composed are quite 

 distinct ; but in the central part they graduate into each other, the Clinton being 

 largely composed of sandstone. There is strong resemblance between the marine 

 vegetation which abounds in the two periods. Westerly the Clinton is more 

 calcareous and more fossiliferous, and graduates up into the Niagara in its litho- 

 logical and fossil characteristics. The Medina, Clinton, and Niagara are clearly 

 defined in some localities; but in others the Medina graduates into the Clinton, and 

 in others the Clinton blends with the Niagara. There is no want of conformability 

 between them where best developed, and the lines of separation show only a 

 changed condition or altered circumstances under which the deposition was 

 continued from one Group to the other. Peiitamerus oblongus, Spirifera radiata, 

 Meristetta cylindrica, and Linguletta lameUata are among the species accredited both to 

 the Clinton and Niagara, and which show the intimate relation between the Groups. 

 The Clinton abounds in fucoids, tracks, and trails, the former being more abundant 

 than in any earlier Group. The fossils having the greater distribution and being 

 most characteristic are Ichnophycus tridactylus, Graptolitfius dintonensis, Helopora 

 fragilis, Athyris naviformis, Leptoccelia hemispherica, Tripleda congesta, Oydon&ma 

 cancettatum, and Comulites distans. The iron ore beds are frequently thick enough 

 to be valuable, and are worked successfully. They are sometimes very fossiliferous, 

 and the quantity of iron is decisive proof of the vegetable character of the 

 fucoids of that age, and the absence of land-plants among the fossils is almost 

 conclusive against their existence at that period. 



CHAPTER XIV. 



NIAQAR.A OROUF 1 . 



103. THIS Group was named from its development at Niagara Falls, where 

 the rock over which the water is precipitated belongs to it. It was defined by 

 Vanuxem in 1842, and by Hall in 1843. It is the most persistent in its geographical 

 distribution of any Upper Silurian Group ; indeed, wherever the Upper Silurian is 

 found it is present, except with the exposed belts of the lower Groups, and not 

 unfrequently it constitutes the whole formation. It generally consists of limestone 

 and shales, but sometimes becomes arenaceous, argillaceous, or highly ferruginous. 

 In New York it exposes an east and west belt almost the entire length of the 

 State, a short distance south of Lake Ontario, with a maximum thickness of 300 

 feet. Near Niagara Falls there are 165 feet of limestone (directly at the falls 85 

 feet) overlying 80 feet of shale. In its western extension it crosses the Niagara Eiver 

 into Canada, appears at Lake Huron, on Manitoulin and Drummond Islands, oc- 

 cupies the southern part of the northern peninsula of Michigan, spreads over the 

 south-eastern part of Wisconsin and the northern part of Illinois. Keeping south 

 of the Lower Silurian area in the north-western part of Illinois, it enters Iowa be- 

 low Dubuque, and presents a surface exposure 160 miles in length by 40 or 50 in 



