214 VEGETABLE STRUCTURE OF COAL. [CHAP. XV. 



of the coal-measures, and consequently come up to the surface all 

 round the margin of the basin. As the dip is usually at a con 

 siderable angle, vertical shafts, from 400 to 800 feet deep, are 

 required to reach the great seam, at the distance of a few hun 

 dred yards inside the edge of the basin. It is only, therefore, along 

 a narrow band of country that the coal can crop out naturally, 

 and even here it is rarely exposed, and only where a river or 

 valley has cut through the superficial drift, often thirty or forty 

 feet thick. The principal coal-seam occurs in greatest force at 

 Blackheath and the adjoining parts of Chesterfield county, where 

 the coal is for the most part very pure, and actually attains the 

 unusual thickness of between thirty or forty feet. I was not a 

 little surprised, when I descended, with Mr. Gifford, a shaft 800 

 feet deep, to find myself in a chamber more than forty feet high, 

 caused by the removal of the coal. Timber props of great 

 strength are required to support the roof, and although the use of 

 wood is lavish here, as in most parts of the United States, the 

 strong props are seen to bend under the incumbent weight. This 

 great seam is sometimes parted from the fundamental granite by 

 an inch or two of shale, which seems to have constituted the soil 

 on which the plants grew. At some points where the granite 

 floor touches the coal, the contact may have been occasioned by 

 subsequent disturbances, for the rocks are fractured and shifted in 

 many places. This more modern coal, as well as that of New 

 castle, and other kinds of more ancient date, exhibits under the 

 microscope distinct evidence of vegetable structure, consisting in 

 this case principally of parallel fibers or tubes, whose walls are 

 pierced with circular or elongated holes. See fig. 5. B. and F. 



By analysis it is found that so far as relates to the proportions 

 of carbon and hydrogen, the composition of this coal is identical 

 with that of ordinary specimens of the most ancient coal of 

 America and Europe, although the latter has been derived from 

 an assemblage of plants of very distinct species. The bituminous 

 coal, for example, of the Ohio coal-field, and that of Alabama, 

 yields the same elements. 



For many years the cities of New York and Philadelphia hava 

 been supplied with gas for lighting their streets and houses, from 



