198 
ANNALS NEW YORK ACADEMY OF SCIENCES 
not been eroded. The faulting may be due to the great disturbance which 
distorted the Glissement beds; the other to an abrupt change in the rate of 
subsidence of the basin. 
The Glissement de L’Esperance. 
The so-called Glissement de l’Esperance, of which the features are 
shown in Plate XVIII, figure 1, and Plate XX, figure 2, is the most notable 
of the disturbances observed. Fayol thus describes and explains it: 
During and after the formation of the Grande Couche, coarse materials, carried 
by the Colombier river, were stopped at the border of the lake, forming some beds 
of pebble rock (poudingue) with steep inclination. At a certain moment, in conse¬ 
quence of the accumulation of plants and mud, the poudingues have slipped, pushing 
before them the mud, not yet consolidated, corroding and folding the vegetal bed. 
In this movement they were turned up in some points so as to dip in direction 
contrary to that of the Grande Couche. After this movement, the sedimentation 
resumed its ordinary course; the irregularites of deposit were effaced by the new 
beds, and when the formation of the Gres Noirs took place, all trace of the great 
Glissement de l’Esperance had disappeared . 1 
It is with no little regret that the writer is compelled to dissent from 
this ingenious explanation as well as from the description of the conditions. 
The statement that after these Glissement rocks were deposited the 
irregularities were effaced by later sedimentation, may be accurate; but that 
cannot be determined now, as no newer beds overlie those of the Glissement. 
The trough now occupied by the light colored sandstones, shales and occa¬ 
sional pebbly layers, was formed long posterior to the deposit of the Gres 
Noirs. A quarry, recently opened at the southerly end of the Longeroux 
trench, shows the trough filled with much the same type of rock as at the 
other end and having as its wall the yelloAV sandstone which overlies the 
black shale of the Gres Noirs. The topography beyond this quarry is such 
as to make probable that in that direction still higher beds occur in the wall. 
It must not be forgotten that at most there remains only the bottom portion 
of this trough, as the region has suffered great erosion; even in recent times 
it has been base-leveled into broad benches. The rock filling the trough 
is unlike any seen elsewhere in the basin, except at the northwest near Cham- 
blet-Neris station. 
The late date at which the deposit was formed makes unacceptable the 
suggestion that it was caused by a slide on the delta slope, or the other that 
its formation has something to do with irregularities in the Grande Couche. 
The natural explanation is that after the lake had been filled, a stream eroded 
1 Fayol, Reunion etc., pp. 35, 36, 37. 
