Foerste.J 
416 • 
f April 6, 
drainage system, somewhat advanced in their development since 
so many hundred meters of Cretaceous and Tertiary strata must 
have been eroded from the crests, flanks, and even in many cases 
from the valleys. Indeed, it can hardly be asserted that all of 
the present synclinal and cataclinal streams had their origin at 
the time of the folding. Since then, many streams having the 
same general direction have probably replaced each other. 
15. Sub-consequent Drainage. As the antecedent and conse- 
quent streams denude the folded surface, the outcropping edges 
of weak strata are exposed, and valleys are developed along the 
strike of these strata, by the headward erosion of branch streams. 
Such streams will here be called sub-consequent. There are 
very few of the smaller streams of the Jura whose courses have 
not been in part modified by subsequently exposed structural 
features. Many streams are sub-consequent along the upper 
parts of their courses, where they occupy valleys cut in the 
softer rocks along the flanks of the folds, bounded on the side 
towards the synclinal valley by some harder and more resisting 
stratum : these are monoclinal sub-consequent streams. Their 
lower courses may be lateral cataclinal streams, of earlier strictly 
consequent origin. Others occupy the crests of the folds and are 
then anticlinal sub-consequents. Both may be formed by the 
headwaters of some lateral stream cutting down deep enough into 
the flanks of a fold to gain entrance to some quite soft stratum, 
and then cutting laterally and undermining the overlying strata 
in a direction more or less parallel to the axis of the fold. Such 
streams are also common in the Jura Mountains, and are thus 
explained by La Roe and Margerie. The natives of the Jura 
are keen observers of topography and have a local word for anti- 
clinal sub-consequent valleys ; they are called combes. 
16. Combes. Many writers have regarded combes as valleys 
originating in long cracks following the trend of the folds, and 
have considered these cracks as due to the splitting open of the 
rocks during folding. This belief finds its expression by J. 
Thurmann, and most subsequent writers ^although L. Rutimeyer 
admitted that aerial denudation alone might be sufficient to start 
combes. A. Heim says that in the upper strata of a fold fractures 
may have opened, but these would not extend to deeper strata 
subjected to greater pressure from superincumbent weight, and 
hence more plastic. 
