70 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



small intermediate deltas. Apparently nothing came from the south, 

 where the waters found their outlet. As the deltas increased in size 

 and approached each other, their elements intermingled. 



The lighter materials, clay and plant, floated into a bay in the 

 southeast corner, where they formed some beds of shale and coal, 

 while in less degree, similar materials floated off on the other side of 

 the Bourrus delta into the bay at the west, where, in like manner, 

 deposits of shale and coal accumulated. Eventually the Bourrus 

 delta divided the lake into two small ponds and in the larger were 

 formed thin irregular lenticular beds of impure coal. At length the 

 lake was filled up and streams began to destroy the coal formation. 

 Disturbances set in afterward but they were not serious, for the 

 Permian deposits are almost horizontal. 



The facts to support this explanation of the origin of the beds, 

 both mineral and vegetable, are presented abundantly in the great 

 excavations. The walls show local faultings, thinning of faisceaux 

 of beds, pebbles of coal are seen in several strata, a great lenticular 

 parting, in part very coarsely conglomerate, occurs in the Grande 

 Couche. This remarkable coal bed is only a few centimeters thick at 

 the southeast outcrop, but it swells thence to 10 to 12 meters and 

 retains that thickness along the outcrop for about 2 kilometers and a 

 half, beyond which it becomes thinner and at length disappears. Fol- 

 lowed down the dip, it decreases in thickness and disappears toward 

 the depth of 350 meters. The outcrop resembles an open C and the 

 interval from the outcrop to the old rock is 500 to 800 meters. 

 Before disappearing at the west, the bed breaks up into six diverging 

 branches. Two other beds, the Gres noirs and the Pourrats, are in 

 contact with the great bed at the southeast but they diverge west- 

 ward. Some lenticular deposits of anthracite occur at the base of the 

 series in both bays. 



Fayol made careful calculation of the quantity of vegetation 

 which could be produced on the whole drainage area of the lake and 

 asserted that enough be produced to give ten times the coal present — 

 and this within the period of 17,000 years. This period is a maxi- 

 mum, corresponding to a very slow filling and to the minimum trans- 

 portation of vegetable material. On the hypothesis of formation in 



70 



