CARBONIFEROUS PERIOD. 329 



selected specimens which were imperfectly burnt (like fig. 562 a), 

 and examined the surface just on the borders of the black portion. 

 Fig. 5G2 b represents a number of ducts thus brought to light, as 

 they appeared when moderately magnified, and fig. 562 c two of the 

 ducts more enlarged ; the black lines being the coal that remained 

 after the partial burning, and the light spaces silica. The ducts 

 were j^th of a millimetre (about four-thousandths of an inch) 

 broad. No stronger evidence could be had of the vegetable origin 

 of anthracite coal. 



Pyrites (sulphuret of iron, page 64) is sometimes disseminated 

 through coal beds in nodules or seams, to the serious injury of the 

 coal. Such coal crumbles down on exposure to the air, and gives 

 forth sulphur fumes when burnt. Even the best of mineral coal 

 contains traces of pyrites ; and to this is owing the sulphur smell 

 ordinarily perceived from coal fires. 



3. Iron-ores. — The iron-ore beds are usually from a few inches to 3 or 4 feet in 

 thickness. They contain the ore in concretionary masses or plates of a stony 

 aspect. The most common hut not most valuable kind has a grayish-blue 

 and drab color on a fresh surface of fracture, and differs from limestone in being 

 unusually heavy : this ore, called clay-ironstone, is an impure spathic iron or 

 ehalybite (p. 63). Another variety of ironstone is an impure hematite (p. 65), 

 affording a red powder. Still another kind is an impure limonite (p. 65), having 

 a reddish-brown or yellowish-brown color and affording a brownish-yellow pow- 

 der : beds of this variety are few, but widely extended, thick, and S-aluable. 



4. Upper and Lower Coul Measures. — The Coal measures are sometimes divided 

 into the Upper and Lower Coal measures. The most convenient division is 

 above the "Mammoth bed" of Pennsylvania, — as there is a marked change in 

 the flora from this point. It has been proposed to make the Mahoning sand- 

 stone the dividing bed, above the Upper Freeport coal bed, which is the third 

 above the so-called Mammoth bed in the Pennsylvania series. Another great 

 sandstone stratum, called the Anvil Rock, occurs in Kentucky, above the twelfth 

 Coal bed in the Kentucky series; and this has also been made a dividing 

 stratum in the measures. There is nothing in the fossils that renders the sub- 

 division at these places of geological importance. (Lesquereux.) 



The great Anthracite region of Pennsylvania is largely Lower Carboniferous. 

 The Upper Carboniferous is present there (at Pottsville, Shamokin, and Wilkes- 

 barre) up to the top of the Pittsburg group (Lesley) ; but the rest does not extend 

 so far eastward. The greatest development of the lower coal was in Pennsyl- 

 vania; and of the upper, in the States farther west. The highest beds in the series 

 appear to occur west of the Mississippi, in Kansas, where they merge into the 

 Permian. There are, however, according to McKinley, 3000 feet of barren Coal 

 measures above the level of the Pittsburg coal, in the southwest corner of Penn- 

 sylvania and the adjoining part of Virginia, and it is not certain how far 

 upward they may reach in the series. 



5. Equivalency of the Aityalachian and Illinois Coal Measures. — There is great 

 difficulty in arriving at safe conclusions as to the equivalency of the beds in 



