a Theory of the Earth . 205 
strata which are below the vegetable earth ; whether the 
pebbles are larger in the deepest strata : nature of the 
rock which forms the solid basis of the valley. 
14. Whether a valley contains foreign pebbles, that is 
to say, which come from the neighbouring mountains : 
to examine to what height they are found on the sides of 
tha mountains ; what may be their origin, and what way 
they may have been conveyed. 
15. In the valleys which contain no foreign pebbles, 
one may follow the traces of those which are there dis- 
covered, and thus ascend to the rock from which they 
Were detached: this has oftenled to curious and useful 
discoveries. 
CHAP, XIII. 
Observations to be made on Tertiary Mountains , or those 
composed of the Wreck of other Mountains. 
1. Whether they do not form the external border of 
other chains of mountains. 
2. Whether, at the extremity of great valleys which 
issue from grand chains of mountains, there are not 
found small hills and even tertiary mountains, which 
seem to have been fbrmed by the accumulation of mat- 
ters deposited by enomous currents that issued formerly 
from these valleys. 
3. Whether their strata do not descend oil the side, 
whence the matter of which they are formed has pro- 
ceeded ? 
4. Size and nature of the fragments, sand and earth, 
of which they are composed. 
5. To observe the order which has been followed in 
the successive deposits of the matters of which they are 
formed. 
6. To compare them with the substances produced by 
