463 
a Theory of the Earth 
the direction of a mountain, or of its strata, when the eye 
is not situated in their prolongation, or at least near it* 
6. The apparent situation of the strata may also lead 
into an error. They appear horizontal even when they 
are very much inclined, and when they are not seen but 
in a section formed by a plane parallel to the common 
section of their planes with the horizon. It is impossible 
to judge of their inclination, and to measure it with cer- 
tainty, but on a section perpendicular to the common sec- 
tion, which I have just mentioned. 
6. A. The greatest error, however, is that which may 
be committed in regard to the super-position of strata. I 
have often seen novices in the study of mountains believe 
that one stratum reposed on another ; one of granite, for 
example, on one of slate $ because they found slate at the 
bottom of the mountain, and granite at the top ; while the 
slate was only laid against the base of the mountain, and 
the granite, on the other hand, was sunk in the earth far 
below the slate. W e must not then say, that a stratum is 
situated below another, but when we really see it extend- 
ing itself below it. 
7. And even when we distinctly see a rock placed above 
another, we must examine whether that which is upper- 
most does not occupy that situation accidentally^ whether 
it has not slipped, or rolled down, from a more elevated 
mountain ; and, in the last place, though they may be 
closely connected, one must examine whether their pre- 
sent situation is really the same in which they were for- 
med, and whether they have not been reversed, and uni- 
ted accidentally in a situation contrary to that of their 
original formation. 
8. One is frequently deceived, also, in regard to the 
nature of stones and of mountains. Though a well-ac- 
customed eye may often judge at some, and even a con- 
siderable distance, of the kind of stone of which a moun- 
