Foerste.J 
396 
[ April 6, 
north of a line connecting La Chaux de Fonds, Courtelary, and 
Orvin, at least six sevenths of the Bernese Jura district being dry 
land. During the Purbeckian epoch, the Bernese Jura district 
was altogether above sea level ; only fresh-water deposits were 
formed in this area. 
It is evident that the drainage during all this period of eleva- 
tion must have been southward into the gradually retreating sea. 
During Cretaceous times various oscillations of the land took 
place. The sea again several times reached the line connecting 
La Chaux de Fonds, Courtelary, and Orvin, but did not pass north 
of the same. But at the close of the Cretaceous and during 
Eocene times all of the Bernese Jura district was once more above 
the sea, and all the drainage of this area, as far as known, was 
still to the southward. 
At present, Cretaceous and Eocene strata are usually found 
only in the synclinal valleys between the folds, the latter showing 
only Jurassic or older formations along their crest. Careful ex- 
amination of cross-sections, however, reveals the fact that Creta- 
ceous and Eocene strata are chiefly confined to the valleys not 
because they are valle} 7 deposits, but because erosion has removed 
these formations from the crests and from the upper portions of 
the flanks of the folds. This is shown by the comparative con- 
formability of the Cretaceous and Eocene strata to the Jurassic 
formations, even along their most elevated exposures on the 
flanks of the folds, proving that these later formations were in- 
volved with the Jurassic strata in the processes of folding, and 
that their present distribution has been determined by subsequent 
events. 
During the Lower Miocene or Tongrian epoch it is evident that 
a low fold involving the greater part of the Bernese Jura ex- 
tended in a general east and west direction, giving rise to a 
second river system in this area. The older series of rivers with 
their general southern course were now largely deprived of their 
headwaters, the place of the latter being taken by a new drainage 
system flowing in an opposite direction, northwards, on the other 
side of the fold, into the Tongrian Sea. Marine deposits of this 
northern sea are found as far southward as a line connecting Por- 
rentruy, Chatillon, and Breitenbach. It will be noticed that it is 
this area which still receives the drainage of those streams of the 
Bernese Jura, which, taking a northward course, are finally col- 
lected in the Birse. 
