STEVENSON, COAL BASIN OF COMMENTRY 
189 
would drain the pond gradually and the whole might be removed by a suc¬ 
cession of floods during a long period of time. This method of cutting 
through landslides is very familiar to American geologists, and it must be 
equally familiar to geologists in Europe, for illustrations are numerous in the 
valley of the Adige and are even better in the valley of the Rhone, where 
vast cones of dejection, which cross the valley, have compelled that mighty 
river to find its way through the thinner part of the mass near the opposite 
wall. These cones have been only trenched by the torrential streams. 
The writer made no study of this deposit and he can offer no explanation 
to account for its presence. 
The tendency to explain conditions by almost cataclysmic agencies 
appears equally in the consideration of the Banc des Chavais. This parting 
in the Grande Couche extends along the outcrop from midway in l’Esperance 
to almost midway in Foret, barely half a mile. It is a lentil, nine or ten 
feet thick at most; it is fine-grained at top and bottom, very coarse in the 
middle; above and below as well as laterally it passes into coal; and its 
longer axis is in direction of the outcrop. This is supposed to be the product 
of a great flood, which tore away the surface of the alluvial plain. The 
coarse material was dropped to form the banc, but only a small part of the 
vegetable matter went down with it; the greater part remained suspended 
being lighter, and subsided slowly afterwards on top of the gross materials, 
so making part of the Banc superieur. 1 
There is difficulty here also. The geographical distribution of the Banc 
des Chavais shows that it was derived from the Colombier region, so that 
it must have been transported by that stream. The fragments of the rock 
are waterworn, not angular, and are supposed to be those left behind on 
the alluvial plain by the Sainte-Aline debacle. If the debacle had left 
fragments stranded on that plain, they could not become waterworn unless 
the plain were covered by moving water — in which case it could not support 
vegetation so abundant as to produce the large amount of coal found in the 
Banc des Chavais. Besides, it is supposed that the Banc de Sainte-Aline 
had swept across this plain only a short time before with its vast mass of 
sand and huge angular blocks, which could not fail to plough off the surface 
after an extraordinary fashion. It is better to seek the explanation in less 
violent action, for one must remember that transition from the Banc inter- 
mediaire to the Banc des Chavais is very gradual at most exposures. 
This matter leads at once to consideration of the mode in which the 
Grande Couche was formed — and this is a matter of chief importance, 
which must be considered in detail. 
1 Favcl: Conimentry, pp. 103, 104. 
