I9II.] STEVENSON— FORMATION OF COAL BEDS. 69 



deltas. ]\Iingled detritus brought in by streams forms a stratified 

 deposit in the basin, where the beds may be composed of a single 

 substance or of several. Those beds are inclined, irregular and 

 of small extent in tranquil waters but less inclined and of wider 

 extent in agitated water. The inclination may vary from o to 45 

 degrees ; different portions of a bed may vary much in age, while 

 beds at different levels may be contemporaneous. The total thick- 

 ness of a deposit has no necessary relation to the sum of thicknesses 

 of the beds which compose it, for a basin, 100 meters deep, may be 

 filled with inclined beds which may have a total thickness of 1,000 

 meters ; he gives illustrations of these conditions. 



The little basin of Ccmmentry is one of several isolated areas in 

 a synclinal which is about 60 kilometers long. These are separated 

 by granite and gneiss and the evidence shows that they were always 

 separate. That of Commentry, 9 by 3 kilometers, contains only Car- 

 boniferous rocks, except at the northwest, where some Permian re- 

 mains. The rocks are not disposed at hazard, but there are definite 

 zones or areas, each with its own type of rock, and these areas, as 

 it were, interlock laterally. Each contains detritus derived from a 

 single locality, though there is a greater or less intermingling where 

 the deposits interlock as overlapping wedges. The history of the 

 basin is thus interpreted by Fayol. 



A lake, 9 by 3 kilometers in area and 800 meters deep, was sur- 

 rounded by steep mountains. Rainwater ate away the surface, 

 digged valleys, carried to the lake pebbles, sands, clay and plant 

 materials, by which at length the lake was filled. This was one of 

 numerous lakes, depressions and alpine elevations on the central 

 plateau of France. Sediment brought in by the streams was heaped 

 up at mouths and formed deltas. The main stream at the northwest, 

 the Bourrus, cut through the mica schist and reached the granite, the 

 latter being found in the upper part of the delta. This delta has the 

 steep slope, with pebbles, blocks, sand, clay and plant debris, all dis- 

 posed in accord with the laws of delta deposit. A somewhat smaller 

 stream, the Colombier, at the east, flowing over anthracitiferous beds 

 and afterwards cutting back to crystalline rocks, formed another 

 delta of similar ty])e ; while petty streams from the north formed 



69 



