172 
SCENERY OF SWITZERLAND. 
nature of the strata, their inclination, and whether 
the fall coincides with, or is in opposition to, that of 
the beds. 
Unless, however, the fall of the ground coincides 
exactly with that of the strata, a river running along 
a transverse valley will generally cross here and there 
harder layers which give rise to cataracts or water- 
falls. 
When the strata are horizontal the action of 
running water is comparatively slow. Steeply in- 
clined or vertical strata on the other hand greatly 
facilitate erosion. Not only does the force of gravity 
take part in the labour, but the water sinks in more 
easily, and both chemical and mechanical disintegra- 
tion is thus much increased. 
Hence it is that while cross valleys often drain 
longitudinal valleys, the reverse seldom happens. 
Cross valleys in fact dominate longitudinal valleys. 
Another respect in which, so far as Switzerland 
is concerned, the longitudinal differ from the trans- 
verse valleys, is that the former run approximately 
east to west, the latter north to south. This makes 
a great difference in their general aspect. In the 
transverse valleys not only do the two sides consist 
of similar rocks, but both receive approximately the 
same amount of light and sunshine, so that the 
