STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 447 



mined for local use. The coal is low in volatile and is said to be 

 an excellent fuel. 



The Coal Measures occupy depressions in gneiss, crystalline 

 schists or in the older paleozoic rocks. The surface on which they 

 were laid down was irregular and Lower Carboniferous is not always 

 present. Within the Zwickau and Lugau areas, one finds the Saar- 

 briick and the Lower Ottweiler, the Upper Ottweiler, if ever present, 

 having been removed by erosion prior to deposition of the Rothlie- 

 gende, which rest discordantly upon the Coal Measures. It is not 

 easy to determine the boundary between Saarbrlick and Ottweiler; 

 Geinitz recognized three zones, marked by Sigillaria, Calamites, 

 Ferns ; later students, however, preferred to make only two. Sigil- 

 laria and Ferns, placing the limit about midway in the Calamites 

 zone. 



The Zwickau area is very small, not more than 20 square miles, 

 but its coal seams are numerous and often very thick. These dis- 

 play in full all the peculiarities of limnic beds, variations in thick- 

 ness, tendency to divide and to subdivide, frequent passage into 

 shale and even into sandstone. The lowest persistent seam is the 

 Planitz, which, at the southwest is practically single, but toward the 

 northeast it is divided by increasing interval rocks and the three 

 main. benches become three seams, A, B, C, each of which has more 

 than one local name. Near Planitz, the thickness is about 10 meters, 

 the interval between A and B being less than half a meter ; nearer 

 Zwickau, the intervals are a half meter and two and a half ; but, 

 farther north, they become 40 and 15 to 30 meters respectively, the 

 coal being 2, 4 and 4 meters in the several seams. The interval 

 rocks are mostly sandstone. Toward the east and south, these 

 seams are broken by so many partings as to be worthless, though 

 they contain much good coal. In great part, the coal is bright Pech- 

 kohle, but it is often laminated or Schierferkohle and at times it 

 passes into Russkohle, in which fusain (Faserkohle or Mineral 

 Charcoal) predominates. The great Russkohlenflotz, at 40 to 56 

 meters above the Planitz, has an extreme thickness of 8 to 9 meters, 

 almost wholly clean Russkohle. Toward the east and north, it 

 breaks up into at least three seams, in which the Russkohle is often 

 replaced with ordinary laminated coal. The coals of this lower 



