413 
[Foerste. 
iMd 
to maintain their positions. At the same time the more elevated 
land areas offered greater slopes and invited greater erosion. With 
increased folding, it must have become less and less possible for 
the weaker streams to maintain their original ground and so they 
then must have sought new channels, or must have ceased to 
exist altogether. The stronger streams might, however, still con- 
tinue their old courses by cutting down the folds gradually arising 
across their path. And so it no doubt came to pass that, as fold- 
ing continued, the slopes offered to erosion became steeper, aerial 
erosion became more marked, all the weaker and medium streams 
ceased to exist, and only the largest and strongest streams ran in 
their old courses by cutting down their gradually rising barriers. 
Theoretically, this process may go even farther. Folding may 
take place too rapidly, or the strata brought up by the folds may 
be too hard, for even the larger streams to cut down their bar- 
riers synchronously with their elevation. The synclinal regions 
between the folds may become marshes and then lakes. Still that 
portion of the old streams which cut across the folds might still 
remain the most ready outlet for the accumulated drainage and 
thus the general direction of the largest and strongest streams 
might be preserved ; but at intermediate points they would be 
represented by marshes and lakes, and only the connecting chan- 
nels cut across the folds would remain to represent any part of 
the drainage which had existed previous to the folding. When 
the channels across the folds had been cut down sufficiently to 
drain the lakes and marshes, it is not likely that the submerged 
channels would be sought out again. Covered by lacustrine 
deposits their history has ceased. This was probably the history 
of some of the Bernese Jura rivers, as will be seen in the latter 
part of this paper. 
Owing to unequal folding, unequal resistance to erosion, or the 
influence of the new drainage systems developed during folding, 
it is, however, by no means certain that those portions of the old 
channels which still remained in existence would still be occupied 
by rivers flowing in the old direction. The direction of drainage 
through these channels might be reversed, or their history as 
drainage channels might even cease, and these beds of streams 
which had existed previous to the folding might remain only as 
“wind gaps.” 
13. Division of the Jura Drainage by recent Warping. It is be- 
