STEVENSON, COAL BASIN OF COMMENTRY 
191 
deposit represents only a bed of four to eight millimeters per century over 
the whole surface of vegetation, or one tenth of the coal material produced, 
* so that nine tenths of the vegetation was decomposed in the air. 
This difference between the amount produced and the amount preserved 
leads him to suggest that possibly the deltas advanced twice as rapidly or 
that the vegetation w T as only half as luxuriant as imagined. In either case, 
the coal material produced would be five times as much as has been pre¬ 
served in the deposits. The brevity of time recjuired by this deposit is 
regarded as one of its chief commendations, for the period assigned to com¬ 
plete formation of the measures is only 170 centuries, whereas the hypothesis 
of accumulation from growth in situ would recjuire 800 centuries for forma¬ 
tion of the coal alone. 1 
The writer cannot see his way clear to acceptance of this explanation. 
One must not forget that the events under consideration occurred, not 
in a great inland sea like Niagara or Erie, but in a little lake, barely six miles 
long and two miles wide; that when accumulation of the Grande Couche 
began, the lake had been almost divided by the Bourrus delta, so that there 
were two ponds, each of which was limited still further by a fringe of new 
land from the other shores; and that from all sides there entered torrential 
streams, emerging from narrow valleys a kilometer to some hundreds of 
meters from the waters edge. The conditions of the larger pond, that of 
Jes Pegauds, are alone to be considered here, as that is the one on which 
chief stress has been laid and to which the writer’s observations were con¬ 
fined. When the Grande Couche was begun, this pond had an area of 
possibly 2,500 acres and was connected with that of les Ferrieres by a strip 
of water along the southern border of the basin. What then must have 
been the conditions on the delta plain and on the highlands ? 
The delta surface is referred to as the “alluvial plain”; but that term 
must be used in this connection with important limitations. When deltas 
are spoken of, one is apt to think of the level deposits at the mouth of the 
Nile, Indus or Mississippi, composed of fine materials brought down by 
streams, thousands of miles long and with gentle fall. Such deltas are 
liable to overflows, but the floods are not violent; in the last stages of sub¬ 
sidence, the floods merely undercut the stream banks, the shores fall and 
throw into the stream whatever may be above. But that was not the con¬ 
dition on the deltas of Lake Commentry. They were made by torrential 
streams, which deposited the coarse material just beyond their emergence 
from the highland, while the finer materials were carried to the water basin 
beyond. One must think of the delta surface as resembling that of dejec- 
1 Fayol: Commentry, pp. 322-324, 330. 
