Coal Dislricl of Si. Etienne. 5 



from 1 to 5 metres [about 3 feet 4 In. to 16 feet 8 in.] in 

 different points these same beds swell out suddenly, so as 

 to acquire a thickness of from 16 to 20 metres [about 53 

 feet 4 in. to 66 feet 8 in."] ; or else not less suddenly they 

 diminish in thickness from the floor and the roof approach- 

 ing each other, to a point at which no traces of them are 

 found for a considerable distance. The latter accident, 

 known in the country by the name of coufflee, occurs more 

 frequently than the other in the coal measures under consi-. 

 deration — this circumstance occasions great inconvenience 

 in the working of St. Etienne collieries, as it throws a great 

 difficulty in the way of tracing the continuity of the coal beds. 



Having thus shewn the most ordinary superposition of the 

 beds in the coal measures, it is necessary to consider the com- 

 mon relations of the different masses. For this purpose that? 

 which occurs when the basin forms a single valley, must be 

 distinguished from that which happens when the basin, 

 greatly dilated, is cut through in different directions by se- 

 veral small valleys. 



Where the coal formation is confined between two con- 

 tinuous and parallel chains, the beds are trough-shaped, and 

 form a new valley enclosed in that of the primitive country. 

 This is the case in the greater part of the Rive de Gier dis- 

 trict, the beds are conformable to the sides of the valley in 

 which they rest, being horizontal or slightly curved at the 

 bottom, and rising rapidly on either side. 



When, on the contrary, and as is generally the case on the 

 watershed towards the Loire, the coal formation is conside- 

 rably dilated, and that it is cut in different directions by val- 

 leys of greater or less depth, it is observable with but few 

 exceptions, that all the beds of the coal measures are inclined 

 in a different direction from the slope of the little isolated 

 hills or terraces formed of them. Thus the outcrops of the 

 beds generally form belts round the hillocks or platforms, and 

 appear on the map as winding lines, differing little in level. 



The coal beds then can, relatively to the portion of the 

 coal measures that occupies us at present, be considered as 

 curved surfaces, tending to form small basins, whose infe- 



