STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 35 



variable, sandy shales change abruptly into coarse massive sand- 

 stone or into clay shale. The shales generally are rich in well- 

 preserved remains of plants, which, according to Fontaine, are 

 allied to those of Virginia and North Carolina. The more massive 

 slates hold silicified stems and branches of shrubs, while the finer- 

 grained sandstones have tree-trunks up to one foot diameter. No 

 false bedding was observed in the sandstones. 



Coal seams are numerous, each prominent slate bed having one 

 or more ; but in all cases these are irregular. Near San Marcial, 

 southeast from the area of detailed examination, much work had 

 been done on supposed anthracite, which proved to be only black 

 slate; but at localities north and northwest from the mining center 

 two seams are known, 8 and lo feet thick. Much of the thicker 

 beds is composed of coal with concentric structure, " shelling out 

 into eggs of greater or less hardness." 



The coal has been affected by the igneous rocks and usually 

 it is a hard anthracite, though occasionally it is coke. In two im- 

 portant openings on the coke, igneous rock is the roof ; in another, 

 it is the floor ; but other pits show no igneous rock anywhere near 

 the coke. In one seam of anthracite, there are pockets of coke near 

 the middle, while in a seam of coke pockets of anthracite were 

 found at the bottom. In several beds divided by partings, coke 

 prevails in some benches, anthracite in others. The proximate 

 composition of the anthracite is : Water, 4 to 8 ; volatile, less than 

 5 ; fixed carbon, 76 to 85 ; ash, 4 to 8 per cent. 



Some Chemical Features of the Coals. 



Coals of various grades are present in the Jura and Trias. 

 Lignite and bituminous coal are present in the Lower Oolite of 

 Great Britain within practically undisturbed rocks and at nearly 

 the same horizon ; while high-grade bituminous coal -prevails in the 

 Lias of Austria and Hungary, where the rocks have suffered severe 

 disturbance. The Lower Lias of Siberia yields high-grade bitumi- 

 nous in the Tcheremkhovo and Grande-Bira fields but typical brown 

 coal in the great Tchoulym region, where the strata are little dis- 

 turbed and the rocks are only slightly consolidated. The Jura- 



