STEVENSON, COAL BASIN OF COMMENTRY 
187 
and les Boulades, were at the west, while the north and west borders were 
furrowed by ravines. The Bourrus delta (Montassiege area) advanced 
most rapidly and reached the southern border of the lake before that of 
Colombier (Longeroux area) had advanced more than one kilometer, and 
the lake was divided into two lakelets, les Pegauds at the east and les Fer- 
rieres at the west; meanwhile, the Bourrus delta had united on the northern 
border with that of the Colombier at the east and with that of the Chamblet 
at the west, so that, within the lakelet areas, one finds a commingling of 
materials from both sides. The coarser detritus was deposited in the 
deltas, but the finer materials passed beyond and were deposited in the lake- 
lets, so that, in the latter, coarse beds are rare, at least in the middle division 
of the measures. These finer materials, both mineral and vegetable, were 
deposited in accordance with their specific gravity, the distribution being 
that observed in river deltas as well as in deltas formed experimentally. 
The area of Pegauds, receiving contributions from both the Bourrus and the 
Colombier, has a much greater amount of mineral and vegetable matter 
than has that of les Ferrieres, which received only from the Bourrus and 
some smaller affluents. 
When the lake had been filled in great part by the transported matter, 
when the Grande Couche had been completed, the Gres Noirs and Pourrats 
coal beds were formed. They are irregular, lenticular, impure and give 
evidence of having been deposited in shallow water, troubled by frequent 
displacement of the mouths of streams. After the formation of the Pour- 
rats bed, no vegetable mass accumulated. At last the lake was filled and 
streams began their work of erosion. There are no horizontal beds, such 
as one would expect to find closing the series in a delta-filled lake — perhaps 
there was none at any time. The absence of these beds is due to later as 
well as to contemporaneous erosion, the latter caused by constant deepening 
of the outlet at the south, amounting in all to about 100 meters. 
The Bourdesoulles area received its coarse materials from les Boulades, 
a gorge with steep walls; the Chamblet stream flowed through a narrow 
valley, also with steep walls, but having a drainage area of five by 2.2 kilo¬ 
meters and resembling the valleys now seen in the region; the Bourrus 
stream was about 15 to 20 kilometers long, with wider drainage area than 
that of Chamblet and was torrential throughout; the extent of the Colombier 
area is not given, but if one may judge from the map, it is supposed to have 
been no smaller than that of the Bourrus and the stream, though torrential, 
was less rapid than the Bourrus. Apparently no debris entered from the 
south. 1 
1 Fayol: Commentry, pp. 63-93. Reunion etc., pp. 20-21. 
