292 ANNALS NEW YORK ACADEMY OF SCIENCES 



But the subsidence became more rapid along the eastern border, the 

 land surface sank below the outlet, the pond was formed anew and with 

 gradually increasing area. Evidently the movement was enough to dis- 

 turb the little streams at the southeast, to render them torrential, so as 

 to sweep out the pebbles, large and small, which had accumulated, as 

 well as to provide a continuing supply. But this condition was of com- 

 paratively brief duration, and one finds above the feldspathic conglomer- 

 ate a recurrence of shales and more or less coarse sandstones. Once more 

 the area is one of muddy flats, but much larger than before. This condi- 

 tion was reached first between the Enne and Eiou Mort, where the marshy 

 growth which was to give the Campagnac bed originated. There the 

 streams from north, east and south had dropped their finer materials and 

 thence the marsh expanded, until it reached the border on all sides of the 

 swampy area. Whether or not it extended to the southeast corner cannot 

 be determined. In any case, the subsidence was irregular in that part 

 of the basin ; the Haute Serre brook was more or less torrential for con- 

 siderable periods, and the advance of the Campagnac marsh across its 

 deposits was interrupted several times. The Haute Serre and Longue- 

 fort streams covered the swamp again and again with more or less coarse 

 deposits, so that the bifurcation of the bed as it crosses the area of de- 

 posit is one of the most interesting features observed in the basin. But 

 this condition had ceased practically at an early period in the history of 

 the Campagnac coal bed. The vest thickness of that bed in the central 

 part of the area and the decreasing thickness in all directions suffice to 

 show that the great marsh had its origin near Cransac. The gentle sub- 

 sidence was long-continued, permitting the accumulation of more than 

 100 feet of coal. 



One would expect to find that so long a period of comparative quiet 

 was followed by one of rapid and extensive adjustment, and in the basin 

 of Decazeville the expectation is fulfilled. Subsidence, still mostly in the 

 east and south, again brought the swampy surface below water and it was 

 covered with a thin coat of fine detritus. The streams became torrential 

 at the east and south, pushing their loads of coarse material into all por- 

 tions of the flooded area. The water was always of moderate depth; 

 frequent shale beds show that the streams were often sluggish, while 

 occasional deposits of coal and coaly material are evidence that here and 

 there mud banks reached to the surface and became swamps. But for the 

 most part, conditions were unfavorable to the development of plants, and 

 mineral deposits continued to a maximum thickness of 750 feet, a mass 

 practically devoid of coal. 



