126 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



whether or not the phenomena at Commentry and Decazeville justify 

 the conckisions embraced in the Delta theory. 



The dips of the strata at Commentry are regarded as all-impor- 

 tant evidence, since in much of the area they compare with those 

 observed in some lake deltas. But it must not be overlooked that 

 the steep dips, 20 to almost 50 degrees, are those in the Pegauds 

 area, in the supposed bay between the Bourrus and Colombier deltas. 

 Before formation of the Grande Couche, the Bourrus delta had prac- 

 tically crossed the basin, dividing it into two little ponds, of which 

 the eastern or larger may have had an extent of rather more than 

 2,000 acres. The coal is on the northern border of this pond or bay, 

 while the outlet was on the south side of the basin. The area of 

 the steep dips is nearly 2 miles from the spot where the Bourrus 

 issued from the mountains and three-fifths of a mile west from the 

 coarse rocks of the Colombier deposits. Its rocks are shales and 

 fine-grained sandstones. It is not on the steep delta slopes, but in 

 the quiet " eddy " between the deltas. The dips in such an area 

 should be gentle, not abrupt. Martins states that, within three-fifths 

 of a mile, the slope of the Aar delta in Lake Brienz decreases from 

 30 degrees to practical horizontality ; De la Beche found the Rhone 

 delta practically horizontal at 2 miles from the shore, while the 

 delta of the torrent of Ripaille, formed in deeper water, showed not 

 more than 10 degrees as the average for half a mile. In every case 

 the decrease is very rapid away from the source of supply and the 

 dip is usually quite gentle within less than a mile, though often very 

 steep at the origin. It would be impossible to explain the steep dips 

 in the Pegauds area, if they be taken as original. But one is not 

 left to surmise in order to explain these dips, for they are not 

 original. 



The cause is clear enough; they are due in chief part to dis- 

 turbance accompanying an outburst of eruptive rock in the northeast 

 corner of Pegauds. This affected not only Pegauds but also the 

 Ferrieres sub-basin, about 3 miles toward the west. This outburst 

 took place when the deposition of Coal Measures rocks had been 

 completed and prior to that of the Permian, which is unconformable. 

 This disturbance crushed sandstones and flexed soft shales between 



