485 
a Theory of the Earth . 
cause some of these parts are each inclosed separately in 
a covering which conceals the interior of them. One 
may guard against this error by observing the fossil in 
the sun with strong magnifying glasses, after having 
moistened its surface with water or the nitrous acid, and 
still better by exposing it gradually to the dame of the 
blow- pipe. 
13. People are often deceived in regard to crystallisa- 
tion, either in the true form of the crystals, or, above all, 
in taking for real crystals parasite crystals, or such as 
have been formed in the moulds made by crystals of ano 
tlier kind. Thus we see crystals of quartz, petro-silex, 
and jasper, formed in the moulds made by calcareous 
crystals, and which have assumed the form of the latter. 
14. In regard to errors occasioned by ignorance of the 
distinctive characters of fossils, and of the names proper 
for them, the only means of avoiding such errors is to 
study with care good authors ; and, above all, collections 
formed, or at least arranged and titled, by able mine- 
ralogists. 
15. But when the slightest doubt is entertained in 
regard to the denomination which ought to be given to 
any fossil, an exact description must be made either of 
its external characters or its most striking physical pro- 
perties, such as weight and solubility.* If this descrip- 
tion is well drawn up, the error respecting the name 
may he rectified, and the observation will not be lost, as 
it would be were there any reason to suspect the justness 
of the denomination, and no means of correcting it by a 
description, f 
* Hardness, refrangibility, electricity, &c. H. 
j: A person now dead, who in his time was considered as a mineralogist, wrote 
t’o me that he had found marine shells in granite. I begged him to give me an 
exact description of the stone which he called granite. He did so ; but I per- 
ceived that the stone was a free-stone or sand-stone, and the specimens he af* 
terwards sent me proved that I was not deceived, We may here recollect Ke 
Yon. I. 3 1ST 
