a Theory of the Earth . %03 
fissure, whether the strata correspond at the same height, 
or whether the corresponding strata are lower on one side 
than on the other. The first case indicates that the fis- 
sure has been produced by mere bursting asunder ; and 
the second proves, besides, a sinking down of the earth.* 
CHAP. XII. 
Observations to be made on the Valleys . 
1. To observe the direction of valleys. Those parallel 
to the chain of the mountains where they are situated are 
called longitudinal ; those which intersect it at right an- 
gles, transversal ; and those which follow an indetermi- 
nate direction, oblique. 
'2. To observe this direction, especially in regard to 
that of the planes of the strata of the mountains. 
3. Dimensions of the valleys ; their length, breadth, 
depth, and the form of their transversal section. 
4. The re-entering and salient angles : whether oppo- 
site to each salient angle, which forms a side of the val- 
ley, the side or opposite mountain forms a re-entering an- 
gle ; or, on the other hand, whether the valley does not 
present alternate constrictions and swellings ? 
5. Whether the opposite mountains correspond by their 
height, their form, the inclination of their corresponding 
faces : the situation of their strata, or their nature ? 
6. Answers to these questions will serve to determine 
whether the valley may or may not be considered as a 
large fissure produced by the bursting asunder of the 
mountains which it traverses. 
7. If the lateral valleys which terminate at a principal 
valley, as the branches of a tree at its trunk, correspond 
* To observe whether this sinking- down has not always taken place on that 
side which looks towards the flat country. Til. 
