399 
[Foerste. 
189.2.] 
eastern folds fork towards the west, thus cansir.g the central 
folds to assume an intermediate position between the east-northeast 
trend of the southern and east trend of the northern folds. 
6. Degree of Erosion of the Folds . — It is evident that the folds 
have been greatly affected by erosion. Tertiary and Cretaceous 
strata are almost invariably removed from the crests and upper 
flanks of the higher folds. It is not until the more resistant beds 
of the upper Jurassic limestones — the Portlandian, Virgulian, Pter- 
ocerian, Astartian, and Corallian — were reached that erosion was 
greatly retarded. This is especially true of the two lower forma- 
tions, the Astartian and Corallian, and in almost equal measure of 
the three upper formations, but at times the Portlandian and Vir- 
gulian are quite strongly eroded. Owing to the more effectual 
resistance offered by these upper Jurassic strata, they now often 
form the crests and the largest part of the flanks of the folds in 
the Bernese Jura, and thus have at times given rise to the opinion 
that the folding of the Jura took place in post- Jurassic rather 
than late Tertiary times. These upper Jurassic strata often form 
topographical features of a different sort, owing to the softness of 
the underlying middle Jurassic shales, belonging to Oxfordian and 
Callovian epochs, when the latter have once been exposed to 
erosion. This may be seen in the monoclinical valleys on the 
flanks of the fold north of Courroux, at Bruchenal and Vorburg, 
also south of Welschenrohr, at Balmberg and near Ruettenen, 
and south of Bassecourt at Joterie. The lower Jurassic lime- 
stones belonging to the Bathonian and Bajocian epochs 
again resist erosion well, and hence they frequently form the 
bottom of the deeper anticlinal valleys, occasionally remaining as 
an anticlinal crest between two lateral middle Jurassic monoclinal 
valleys, whose outer margins are composed of upper Jurassic 
limestones, as may be seen on the Chasseral, north of Nods. 
However, although a good amount of erosion has been accom- 
plished, it must be remembered that the ridges of the Jura are of 
anticlinal structure, and that they present a much closer degree of 
correspondence between structure and form than is common in 
mountainous regions. 
ORIGIN OF THE JURA DRAINAGE. 
7. Previous Theories hearing on this Subject. — If a geologist 
should take up a good topographical map of the Bernese Jura, 
