1(64 Hints for the Formation of 
tain is composed, such judgment is however often er 
roneous : mountains of granite, or gneiss , tender and 
destructible, often assume, at a distance, the round form 
of secondary mountains ; sometimes, also, mountains of 
calcareous stone, hard of their kind, and in strata either 
vertical or very much inclined, present the bold forms, 
the peaks, and sharp-angled indentations of the granite 
summits. 
9. People are often deceived even on a near view. A 
stone may have a foreign covering of mica, for example, 
while the interior part is of a very different nature. 
10. Effervescence with the nitrous acid is commonly 
considered as a certain character of calcareous stone ; but 
this character may be deceptive, since barytes and mag- 
nesia effervesce also and we must not consider it 
enough to touch a stone with the nitrous acid, or to let 
fall a drop of the acid on its surface, since the absorbing 
earth, whatever it is, may be only disseminated between 
argillaceous or siliceous particles. We must therefore 
immerse a fragment of the stone in a quantity of the acid 
sufficient to dissolve it entirely, if it be wholly soluble, 
and observe whether there remains any residuum that 
withstands solution. 
11. The action of the air and of meteors often gives 
fossils appearances absolutely different from those which 
they had before they were subjected to it. We must not 
then be satisfied with a superficial examination : we must 
sound the rocks to the quick where the action of meteoric 
agents has not penetrated. 
IS. People are often deceived, also, in considering 
compound stones as simple stones, when the composition 
of them does not manifest itself on the first view, either on 
account of the smallness of their composing parts, or be- 
* And, on the other hand, there are calcareous stones which do not effervesce. 
