76 COAL MEASURES. 



marshes were subject to overflows, as shown by the remains of fish and beds of 

 sand and shale, while land-shells, air-breathing reptiles, and trees show the presence 

 of laud. The bark of the trees was the [most durable part, aud it is not un- 

 usual in sandstone to find only a cast of the tree, covered with a thin film of coal, 

 retaining the original markings of the bark. Some blocks of coal are composed of 

 thin layers formed from the bark of trees and nothing else. Beds vary in purity, 

 from coal with less than one per cent of earthy matter to dark -colored shales, with 

 only a trace of coal. 



§ 164. When bituminous coal has lost part of its hydrocarbon gas, it is semi- 

 bituminous, as at Blossburg and Broad Top Mountain coal-fields in Pennsylvania ; 

 but if the bitumen is all driven off, it is converted into anthracite. At gas- 

 works bituminous coal is put in a retort, and by the application of heat the gas 

 is driven ofT, leaving a residue of coke; but if the gas is driven off under great 

 pressure, the residuum is anthracite. When coal melts and runs together in the 

 fire, forming a crust which must be broken to give vent to the draft, it is cok- 

 ing coal. Splint-coal or block -coal does not melt and run together, and is there- 

 fore dry-burning coal. Cannel-coal burns with a bright flame like that of a 

 candle, from which circumstance it derived its name. Cannel was the pronuncia- 

 tion of caudle in Scotland and England, where this coal received its name. Coal 

 containing sulphur is unfit for smelting iron ores in a blast-furnace, and is not 

 suitable for the manufacture of illuminating gas. 



§ 165. Bituminous shales frequently contain iron ore disseminated through 

 them, either as a carbonate or sesquioxide, and sometimes forming black-bands. 

 The same layer of shale which constitutes black-band ore at one place will 

 have the ore gathered in balls, arranged in rows, at another place. By chem- 

 ical affinity the disseminated particles were brought together, and formed into 

 balls or discs ; aud hence the iron exists in all stages, from fine distribution 

 through the shales to layers of kidney ores, with whitened shales intervening. 

 The iron ores of the Coal Measures are generally hardened mud, charged with 

 iron, or clay-iron stone, and rarely yield more than 40 per cent of iron, and they 

 are not of much value except as they exist around the margin of the Appalachian 

 coal-field in the Lower Coal Measures. No good iron-mines are found in the 

 other coal-basins. The greater part of iron manufactured from these ores has been 

 obtained in Pennsylvania. 



§ 166. The first trace of reptiles observed in the Carboniferous System con- 

 sisted of foot-prints, found in 1841, in the Lower Coal Measures of Horton Bluff, in 

 Nova Scotia. This was followed in 1844 by the discovery of reptilian bones at 

 Saarbruck, and in 1851 to 1853, bones in Nova Scotia, and the laud-snail, Pupa 

 vetusta. Since that time the discoveries have been numerous. There is no reason 

 to suppose the atmosphere was charged then with auy more carbonic acid than it 

 is now; on the contrary, the air-breathing animals prove it was not. The life of 

 plants and animals is controlled by oxygen, and the adaptation of organs is in ac- 

 cordance with its properties. If there was less oxygen in the atmosphere, the mem- 

 branous reptile lung could not supply the demands of its system, and analogy proves 

 these animals could not have existed in the coal period with a less proportion of 

 oxygen than is required now. 



§ 167. The coal-beds and the vegetation of the coal period are usually suffi- 



