Foerste.] 
394 
[April 6. 
La Noe et Margerie. Les Formes du Terrain. 1888, pp. 140-143, 157, 
158. 
A. Penck. Die Bildung der Durchbruchthaler. 1888, p. 45. 
Prefatory Note. — The following studies were begun at 
Harvard College in 1890, for a thesis in a second course in physi- 
cal geography. The work was then based on the excellent topo- 
graphical and geological maps of the Bernese Jura, and was carried 
on under the direction of Prof. W. M. Davis ; it was afterwards 
continued in the field by the writer. For most of the suggestions 
and theories herein embodied, and for all the training and 
incentives to work, he is directly indebted to Professor Davis, to 
whom he desires here to make full acknowledgments and to render 
his warmest thanks. 
1 . Introduction . — In a study of the drainage of mountain systems 
which are the result of folding, it is obviously of advantage to 
secure a clear idea of the initial streams consequent upon folding 
in any area. In a typical case the drainage existing antecedent 
to folding should have no visible influence upon the location of 
consequent streams, and such antecedent streams should them- 
selves sometimes succumb to the changed conditions brought 
about by the folding, and cease to exist. To avoid unnecessary 
complications in the problem, a region should be chosen where 
the folds are not overturned ; faults, volcanic intrusions, and all 
mountain building forces other than folding should be practically 
absent ; and subsequent erosion should not have been sufficient to 
carve out paths for the initial consequent streams greatly at 
variance with the courses first offered by the folds. Finally, the 
topography previous to folding should have been as little accentu- 
ated as possible. 
The Appalachians, although an acknowledged type of folded 
mountain systems, do not fulfil the conditions above enumerated. 
The crests, flanks, and troughs of the folds are strongly eroded, 
and the varying degrees of resistance to erosion offered by the 
strata thus laid bare have given rise to the formation of new 
drainage channels whose origin and direction are due to causes 
subsequent to the development of the initial drainage that was 
at first consequent on the folding. 
The Jura Mountains offer some advantage over the Appalachians 
in the study of drainage in that the erosion subsequent to folding 
