LOG 
MOUNTAINS AND THEIR ORIGIN. 
points on the opposite sides, so that, in travelling 
through such a transverse valley, you turn to the 
right or left, as the case may be, where it enters 
the longitudinal valley, and follow that till you 
come to another transverse valley opening into it 
from the opposite side, through which you make 
your way out, thus crossing the chain in a zigzag 
course (as in Fig- 
ure 5). Such val- 
leys are often much 
narrower at some 
points than at oth- 
ers. There are 
even places in the 
Jura where a rent 
in the chain begins with a mere crack, — a slit 
but just wide enough to admit the blade of a 
knife ; follow it for a while, and you may find 
it spreading gradually into a wider chasm, and 
finally expanding into a valley perhaps half a mile 
wide, or even wider. 
By means of such cracks, rivers often pass 
through lofty mountain-chains, and when we 
come to the investigation of the glacial phenom- 
ena connected with the course of the Rhone, we 
shall find that river following the longitudinal 
valley which separates the northern and southern 
parts of the chain of the Alps till it comes to 
Martigny, where it takes a sharp turn to the 
