STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 43 



5 per cent. ; but in two mines the samples represent different parts 

 of the great seam and the contrast in conditions is marked; at one 

 mine, the ash content in the several parts is, ascending, 10.82, 5.10 

 and 9.52 ; in another, the percentages are 5.20, 22.20, 9.80 and 

 22.60. 



There is little of detailed information respecting variations of 

 coal in different portions of lenses, as analyses have been made only 

 of coals supposed to be worth mining. But incidental references 

 abound, which show that, toward the borders, ash increases until 

 the coal becomes worthless. 



Summary. 



The areas of Jura and Trias, containing coal in economic quan- 

 tity, are utterly insignificant, when compared with those in which 

 the systems are exposed ; but there are many localities in which coal 

 accumulated during brief periods and amid unfavorable conditions. 

 The oolite coals of Britain and a few spots on the continent of 

 Europe are of inferior quality, merely local and almost without 

 interest. Elsewhere the useful deposits are in the lower part of 

 the Lias and in the highest divisions of the Trias. The Jurassic 

 above the Lias and the Triassic below the Keuper may be regarded 

 as barren. 



The associated rocks are as in the later periods. The Oolite 

 coals of England are intercalated in sands ; the Jurassic coal of 

 Spitzbergen is confined to the Middle or sandstone division, as de- 

 fined by Nathorst; the Grestener or coal-bearing Lias of Austria 

 is composed of sandstones and clays ; the same conditions prevail 

 in the Liassic coal areas of Hungary and Siberia ; the Jura-Trias of 

 Queensland and New South Wales are almost wholly sandstone ; the 

 upper Trias in Austria and Hungary is sandstone with intercalated 

 shale. But the Jura in Alaska is almost wholly shale and the Upper 

 Trias in some small areas has little sandstone. Freshwater fossils, 

 in rocks associated with coal seams, have been oberved in England, 

 Siberia, Spitzbergen, France and Quuensland. The structure of the 

 rocks is evidence of, at most, shallow water and in some cases it is 

 very suggestive of eolian agency. False bedding is reported from 

 England, Australia, Germany and North Carolina and ripple marks 



