397 
[Foerste. 
1S92.] 
This condition of things seems, however, to have been of short 
duration, since in the Middle Miocene or Delemontian epoch the 
northern sea had again retreated, leaving the Bernese Jura dis- 
trict once more entirely above sea level. 
On the other hand the southern sea seems to have encroached 
upon the land again, and during the Upper Miocene or Helvetian 
epoch, marine deposits were formed over southern and central 
districts of the Bernese Jura, as far north as aline connecting La 
Chaux de Fonds, Tremelan-dessus, Glovelier, Courchapoix, and 
the mountain fold south of Numingen. At the same time fresh- 
water deposits seem to have been laid down over the area north 
of the line just described. The fresh-water deposits of the north- 
ern Bernese Jura area owe their origin perhaps to large ponds and 
lakes in this region, indicating a depression of land here, the 
remnant, it may be, of that depression of the northern Jura 
which permitted the digression of the sea upon the northern Hank 
of the Tongrian fold, as already described. During the Oeningen 
epoch these fresli-water deposits extended farther southward ; in 
fact, crossed the Jura and reached central Switzerland. 
3. Incomplete Evidence of Land Conditions . — It will be noticed 
that the limitations of the sea deposits as at present preserved 
have been the sole basis for the boundaries assigned to the sea in 
former epochs. In the same way the absence of such sea deposits 
has been considered sufficient evidence of the dry land conditions 
of the corresponding areas, during the .period indicated by the 
hiatus in the regular succession of marine sedimentation. Such 
evidence includes several sources of error. Manifestly, the only 
safe means of proving the existence of land conditions in any area 
is to bring proof of a positive rather than of a negative character ; 
for example : shore lines, river courses, irregular erosion of sub- 
aerial nature. So far, the reports on the Swiss Jura here con- 
sidered do not contain statement of positive proofs of land con- 
ditions, and hence the preceding r6sum6 is based almost entirely 
upon evidence as furnished by marine deposits. 
4. The Period of Folding . — The lower part of the Oeningen 
deposits contains many conglomeritic facies. The pebbles com- 
posing the same are often evidently of igneous origin and must 
have been derived from different sources. Their lithological 
characteristics associate them with the igneous rocks found in situ 
