STEVENSON, COAL BASIN OF COMMENTRY 
177 
Descending to the bottom of the trench, where the Grande Couche is 
exposed, one finds the gray sandstone of the Gres Noirs more than 20 feet 
thick, containing a thin irregular lentil of coal and coming down to within 
10 feet of the Grande Couche. A deep pit at a few rods south from this 
place shows a face of about 30 feet of coal, with the bottom not reached. 
Here the gray sandstone is almost in contact with the coal bed. 
The Grande Couche has been subjected to pressure severe enough to 
break it into great wedges and a shale belonging above the coal was involved 
in the disturbance. Plate XIX, figure 2, was taken obliquely, so as to 
embrace some other features and it does not give the details of structure in 
the coal as sharply as is desirable. An attempt is made in Figure 2 to indicate 
some features less sharply shown or concealed in the view. There are three 
lines of fracture; one inverted wedge projects above the general surface of 
the bed; the recumbent wedge at the right has had an irregular under 
FIGURE 2. THE GRANDE COUCHE IN TRANCHe'e DE LONGEROUX. 
surface and it breaks up near the apex, so that two separated fragments are 
in the shale beyond. This shale is closely folded on itself — a detail not 
fully shown in the photograph — and this fold involves the coal also, for a 
prong of crushed coal, shown indistinctly in the view, passes down into the 
bed. The relation of the overlying dark shales is shown distinctly; they do 
not share in the disturbance which affected the coal and its accompanying 
shale. The fold in the bottom sandstone of the Gres Noirs is shown in the 
upper part of the photograph, below the tree. 
The manner in which this coal is broken, the sharpness of the lines bound¬ 
ing the wedges, the acuteness of the projecting apex and the crushed frag¬ 
ments within the loop leave no room for doubt that the disturbance, what¬ 
ever its nature may have been, occurred not while the vegetable matter was 
in pulpy condition but after it had been consolidated — after it had been 
converted into a bed of coal. The shales involved in this crush are unlike 
those between the coal and the Gres Noirs, which are the dark shales and, 
along this outcrop, are undisturbed, though they show irregularities along 
the dip — not related to those of the coal as exposed here. 
The Banc des Brouillages, or Banc du mur, is well exposed as it forms 
the easterly wall in a great part of the excavation. 
