DESCRIPTIONS OF THE FORMATIONS. 31 



into thin laminae after long exposure. In many places this rock contains so much 

 bitumen as to give out flame when thrown upon a fire of hot coals. In Western 

 New York it is fifty feet thick, and farther east much thicker. H. 



This important formation carries its broad black outcrops across many of the 

 Middle and Southern States, with comparatively little change, but in the South the 

 black shale is supposed to be Genesee. In the Juniata region of Pennsylvania 

 the Marcellus has been found to measure 875 feet thick, and is there divisible 

 into an upper, middle and lower member, the last consisting of black and brown 

 shales, the surface being stained with iron rust, &c., coated with bituminous 

 matter. In Perry County, Pennsylvania, small coal beds occur in this formation, 

 constituting the oldest known coal-measures, and significantly marking the great 

 change in the general condition of things which either followed or was introduced 

 by the deposit of the Oriskany sandstone. Lesley. 



In speculating upon the origin of petroleum, some geologists have sought it 

 in a process of distillation from the black Marcellus and Genesee shales upward, 

 and of condensation in the oil-bearing gravels and fissures of the overlying 

 formations. Chemists, like T. Sterry Hunt, oppose this view on chemical grounds, 

 others oppose it from other considerations of apparently equal weight. It is a 

 curious fact, however, that at this horizon, and in the Upper Helderberg or 

 Corniferous, occur the petroleum deposits of Upper Canada, while the Pennsyl- 

 vania oil-deposits lie at successively higher and higher stages hi the series. 



10 b. Hamilton, This group takes its name from the town of Hamilton, hi 

 Madison County, New York, which contains no other rock, and where the best 

 opportunity exists of examining the members of which it is composed, and where 

 its fossils are in great abundance. It includes all the masses between the upper 

 shales of Marcellus, and the Tully limestone, and is from 300 to 700 feet hi 

 thickness hi New York. It is important from its fine agricultural qualities, its 

 thickness and extent, commencing at the Hudson and extending to Lake Erie. 

 It consists of slate, shale and sandstone, with endless mixtures of these materials, 

 or, in other words, sandy shale and shaly sandstones, and 'is not very easily 

 described. There are three distinct mineral masses as to kinds, but not as to 

 arrangement. The first, in the order of the tenuity of particles, is rather a fine 

 grained shale, often fissile or slaty, its color some shade of blue, usually dark or 

 blackish. The second is a coarse shale, often mixed with carbonate of lime, its 

 color blue or dark gray when fresh, but becoming of an olive or brown color by 

 long exposure to the weather, the color being due to manganese. It has no tendency 

 whatever to separate into regular layers, but when a mass has been long exposed it 

 shows numerous curved divisions, the curves very short and irregular, giving it a 

 very peculiar appearance, which is unmistakable. The third kind, which is not so 

 common as the two first, is a well characterized sandstone, and is generally in the 

 upper part of the group, but more or less mixed with either of the two others. It 

 is often in layers, though rarely straight, and usually short, interrupted, sometimes 

 mixed with carbonate of lime. The colors of this kind are of more various 

 shades, olive, greenish and yellowish. One thin layer produces excellent flagstones, 

 but the group generally is deficient in building materials, the shale of the first kind 

 readily crumbling by exposure to the air ; the two latter kinds alone furnishing 

 building stone. The best is where limestone forms the cement, and sand is hi the 



