390 PALAEOZOIC SYSTEM OF ROCKS. 



Cellulose C36H 6 o0 3 o 



Subtract -j gnu 2 [ C23H32O30 



And there remains Ci 3 H 28 = petroleum. 



Origin of Varieties. — However formed, there can be no doubt that 

 the different varieties of this series are formed from one another by a 

 subsequent process. It is certain that from all varieties 0H 4 is con- 

 stantly passing off, and that the result of this is a slow consolidation. 

 By this process light oil is changed into heavy oil, heavy oil into bitu- 

 men, and bitumen into asphalt. Some of the grandest fissure-reservoirs 

 of oil have thus been changed into solid asphalt. In the upper barren 

 Coal-measures of West Virginia there is a vein of asphalt four feet 

 thick, over 3,000 feet long, and of unknown depth. It fills a great 

 fissure which breaks through the rocks nearly perpendicularly, and out- 

 crops on the surface. 



There are, therefore, two series of substances formed from . organic 

 matter, viz., the coal series and the oil series. In each series the pro- 

 portion of carbon increases by subsequent change until, perhaps, pure 

 carbon may be reached. In the coal series we have fat coal, bitumi- 

 nous coal, semi-anthracite, anthracite, and, finally, graphite. In the oil 

 series we have light oil, heavy oil, bitumen, asphalt, probably jet, and 

 possibly, finally, diamond: for Liebig has suggested that diamond is 

 most probably formed by crystallization of carbon from a liquid hydro- 

 carbon, in which the proportion of carbon is constantly increasing by 

 loss of CII 4 .* 



Future of this Industry. — The oil in the United States is practically 

 inexhaustible. The finding of great reservoirs, producing spouting- 

 wells, has always been, and always will be, very uncertain, and the 

 duration of their productiveness limited ; but a moderate return for 

 industry and capital is certain for an almost unlimited time. A large 

 portion of the PalaBozoic basin, including an area of about 200,000 

 square miles, is underlaid by rocks which are more or less oil-bearing. 

 The eastern portion of the United States is the great oil-bearing, as it 

 is the great coal-bearing, country of the world. The gas supply will 

 probably be much more quickly exhausted. 



Fauna of the Carboniferous Age. 



As heretofore, we will disregard the subdivisions, and treat of the 

 fauna of the whole age, or at least the two periods sub- Carboniferous 

 and Carboniferous, together. It must be borne in mind, however, that 

 most of the lower marine animals mentioned are from the sub-Carbon- 



* This view seems to be confirmed by recent observations in South Africa and South 

 America. Lewis, Science, vol. viii, p. 345 ; Derby, Science, vol. ix, p. 57. 



