a@ Theory of the Earth. 139 
6. A. The greateft error, however, is that which mdy 
‘be committed in regard to the fuper-pofition of firata. I 
have often feen novices in the ftudy of mountains believe 
that one ftratum repofed on another; one of granite, for ex- 
| ample, on one of flate; becaufe they found flate at the bot- 
tom of the mountain, anid granite at the top; while the flate 
was only laid againft the bafe of the mountain, and the granite, 
‘on the other hava: was funk in the earth far below the flate. 
We muft not thei! fay, that a ftratum is fituated below ari- 
other, but when we really fee it extending itfelf below it. 
7. And even when we diftin&ly fee a rock placed above 
another, we muft examine whether that which is uppermoft 
does not occupy that fituation accidentally ; whether it has 
not flipped, or rolled down, from a more elevated mountain ; 
‘and, in the laft place, though they may be clofely connetted, 
one muft examine whether their prefent fituation is really 
al Sen! 
. 
‘the fame in which they were formed, arid whether they have 
‘not been reverfed, and united accidentally in a fituation con- 
trary to that of their original formation. 
8. One is frequently deceived, alfo, in regard to the nature 
of ftones and of mountains. Though:a well-accuftomed eye 
may often judge at fome, and even a confiderable diftance, of 
the kind of ftone of which a mountain is compofed, fuch ° 
_ judgment is however often erroneotis: mountains of granite, . 
or gneifs, tender and deftruétible, often affume, at a diftance, 
the round form of fecondary mountains; fometimes, alfe, 
‘mountains of calcareous flone, hard of their kind, ‘and iii 
- fata either vertical or very much inclined, prefent the bold 
forms, the peaks, and fharp-angled indentations of the gra- 
nite fummits. 
g. People are often deceived even on a hear view. A ftone 
may have a foreign covering of mica, for example, while the 
interior part is of a very different nature. 
to. Effervefcence with the nitrous acid is commonly con- 
fidered as a certain character of calcareous ftone; but this 
charaéter may be deteptive, fince barytes and sided a ef- 
fervefce alfo*: and we muft not confider it enough to touch 
* And, on the other hand, there are calcareous ftones which do nét 
‘CFfervefee, j 
Vor. V, Lc a ftone 
