30 THE GEOLOGIST'S TRAVELING HAND-BOOK. 



9 d. Corniferous Limestone. For all practical purposes, this and the Onondaga 

 limestone may be regarded as one formation. It extends from the Hudson River 

 to the Niagara River, which it crosses at Black Rock, producing there a rapid 

 current at the International Bridge, at Buffalo, and forming a small island just 

 above the water. It extends far into Canada, is seen at Sandusky City, Ohio, and 

 there forms the bottom of Lake Erie. Its color varies from a light grayish-blue 

 to a black, and is sometimes even a light gray or drab. It contains numerous 

 nodules of flint or hornstone, from which it derives its name. But few if any of 

 the layers afford a pure limestone. Its color varies from black to gray, brownish 

 and light blue. It is usually in regular courses from six to eighteen inches thick, 

 separated by layers of hornstone, and sometimes embracing flattened nodules of 

 the same. This rock is crossed by vertical joints in two directions, giving rise to 

 numerous copious springs of water. An upper division, called the Seneca limestone, 

 is now included in the Corniferous . In New Jersey and Pennsylvania it is a blue 

 and sometimes sparry limestone, including bands and nodules of chert. In Canada 

 and the Western States it is a straw-colored and light gray rock. In its general 

 eastern exposures it is generally bluish. Above the Corniferous are no general 

 limestone masses in the Eastern States, but partial deposits only, the most ex- 

 tensive of which is the Tully limestone, found only in Central New York. There is 

 an astonishing change from the top of the Corniferous limestone to the black 

 shales of Marcellus. Two formations more unlike cannot anywhere be found. 

 Both the Corniferous and Onondaga are included hi the Upper Helderberg lime- 

 stone of Pennsylvania, and on the Juniata they measure together only sixty feet. 

 Immediately upon the upper surface of the Corniferous limestone, lies the valuable 

 and extensive MABCELLUS IRON OEE. This consists of carbonate of iron, which 

 occurs ha a bed of pyritous clay, and near the outcrop is changed into limonite. 



10. MIDDLE DEVONIAN. 



10 a. Marcellus Shales are of a black color, usually dark brown when 

 altered. They greatly resemble the Utica slate in mineral character, and could 

 readily be mistaken for it. They extend in New York from the Hudson River to 

 Lake Erie. The lower part contains some impure black limestone, not in layers 

 or beds, but in interrupted flattened masses. The upper shales are not so highly 

 colored as the lower ones, and are disposed to separate, when long exposed, into 

 small, thin-edged fragments, the result of a peculiar accretionary structure. The 

 fragments often exhibit stains, in spots, from iron rust, and also minute crystals 

 of gypsum, the effect of the action of decomposed pyrites and limestone particles. 

 Some portions of the lower shales are black and friable from email carbonaceous 

 f ucoids. Along the whole line of its outcrop it has been dug into in vain attempts 

 to find coal. Van U. 147. It has two joint planes, nearly at right angles to each 

 other, causing projecting corners of rock, with smooth nearly vertical surfaces. 

 These are sometimes seen in the upper members also of the Hamilton group, and 

 the septaria or flattened balls of black limestone also occur in the Genesee shales. 



The lower part is very black, slaty and bituminous, and contains iron pyrites 

 in great profusion. In general character the lower part resembles the Utica slate 

 and is not distinguishable from the 10 c. Genesee slate, in its general aspect. "When 

 long exposed, the lower part weathers to a brownish or iron-rust color, partly 

 from the presence and decomposition of iron pyrites and partly from bituminous 

 matter. In some situations it retains its purely black color, and scarcely separates 



