STEVENSON, COAL BASIN OF COMMENTRY 
185 
open “C,” has its greatest thickness along the northern and northeastern 
border; it thins out to nothing on the eastern side, but on the western side 
it divides into several branches, each of which fades away as the sandstone 
wedges thicken, until this sandstone becomes a continuous section. Down 
the dip, the bed gradually decreases and ends in several thin beds of car¬ 
bonaceous shale. 
History of the Commentry Basin. 
Such are the facts observed within that part of the Pegauds area which 
contains the Grande Couche and its subdivisions. It remains to consider 
the succession of events in the area, and this must be done with a degree of 
detail, justified, not by any importance possessed by the area itself, but bv 
the importance of generalizations which have been based on its structure. 1 
The deposits in the Commentry basin have been divided into 
Lower, consisting almost wholly of rather coarse materials, but containing 
some unimportant lentils of anthracite; 
Middle, consisting chiefly of fine materials and including the great coal beds; 
Upper, containing practically no coal and consisting of rocks more or less 
coarse. 
Only indirect reference has been made to rocks of the Lower division, as 
they seemed to have little bearing upon the problems with which the writer’s 
study is concerned; but it is well to note the conditions as described by 
Fayol. 
The total thickness of beds between the Grande Couche and the bottom 
is 800 meters at the eastern extremity and 500 meters back from the Tranchee 
de Foret. These thicknesses were ascertained by surface measurements, 
as no boring to the bottom rock has been made under the coal bed and 
nothing is known respecting conditions midway in the Pegauds. These 
Lower beds have little coal, but they yield ample evidence that, before they 
were deposited, vegetation had gained hold on the surrounding country. 
For the most part, the detritus is coarse and shale is of rare occurrence. 
One singular deposit, the Banc de Sainte-Aline, only 15 meters below the 
Grande Couche, has a curved outcrop similar to that of the coal bed and 
is a mass of mica schist, granulite and granite fragments, many of them very 
large, mostly angular and cemented by smaller fragments of the same rocks. 
Its extreme thickness, 60 meters, is on the northerly and northeasterly border 
1 Some important conclusions presented in Fayol’s work, as well as ingenious suggestions 
offered by several later authors cannot be considered here. They will be discussed in a mono¬ 
graph upon the formation of coal beds which the writer has in preparation. 
