STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 471 



in part upon 4 and so has only 15 seams. The number of workable 

 seams does not exceed 77 and even that estimate may be excessive. 

 The zone of Flines, equivalent to Andenne of Belgium, gives evi- 

 dence of at elast five invasions by the sea. 



The Concession of Dourges was studied many years ago by Bre- 

 ton.'^" He recognized a general decrease in volatile downward in 

 the section, but the change is not in accordance with an exact law, 

 for it is true only of seams far apart, not of those near together. 

 Similar variation is observed in a seam, when followed for a con- 

 siderable distance. The roof in each case has its own plants along 

 with others not peculiar to it. The exposed section in southern 

 Dourges is about 750 meters thick with 80 coal seams, measuring 

 from one centimeter to a meter and a half. The area is greatly dis- 

 turbed by folds and faults. 



There are ^6 beds of sandstone, the thickest being 22 meters. 

 They vary greatly but not abruptly and consist of quartz grains with 

 clay and some mica. Occasionally, they contain pockets of bright 

 coal, and trunks of trees are not rare. Sandstone, at times, replaces 

 a coal bed, though the mur and toit persist in such cases. Shale 

 in roof of a coal seam is darkest near the coal but the best impres- 

 sions of plants are at about a half meter above. He notes one 

 marine deposit, about 7 meters thick, containing many specimens of 

 Productus and Orthoceras. 



Breton groups the coals into grasses, which ignite readily, are 

 rich in gas, fuse well, give off dense smoke and leave a white ash, 

 and seches, less easily ignited, burn slowly, give less smoke, do not 

 agglutinate and leave a reddish ash. These often have much min- 

 eral charcoal, which bears close resemblance to wood charcoal. 

 Coal seams usually have shale at top or bottom or as partings, which, 

 in the fat coals, is combustible and is used as fuel for the boilers or 

 is given to the poor. He emphasizes the fact that, very often, there 

 is a veinette near a thick seam, with which it is apt to unite. 



He groups the deposits into faisceaux. The highest is that of 

 the charbons tres-gras, shown in the eastern part of the Concession. 

 This, about 300 meters thick, has 7 workable seams with 6.15 meters 



• ^0 L. Breton, " £tude geologique du sua de la concession de Dourges,"" 

 Soc. dcs Sci. Lille, 1872, pp. 355-422. 



