44* Count de Bournon's Description of a 
dony; but, from the variety of forms they have assigned to 
these crystals, (which forms have no connection with each 
other, and evidently belong to other known substances,) it is 
very clear that all these pretended crystals of calcedony, have 
been produced merely by its having taken the place of other 
substances; either by being moulded in the cavities left by 
the destruction of the crystals of such substances, or by being 
formed at the very instant of the destruction of those crystals, 
by an operation analogous to that known by the name of ce- 
mentation. Examples of the nature here spoken of, are well 
known to happen in many other substances. Besides, those stones 
which offer the greatest resistance to a mechanical division, such 
as quartz, blue corundum or sapphire, zircon, garnet, &c. some- 
times show, by fortunate accidental fractures, or other natural 
accidents, the direction of the crystalline laminae of which they 
are composed. Quartz, for instance, is sometimes found with an 
evidently laminated appearance, particularly the blue variety, 
which is brought to us either from Canada or from the East 
Indies ; but never yet was an appearance of this nature perceived 
in any kind of calcedony. 
Many celebrated mineralogists, amongst whom may be men- 
tioned the Abb4 Hauy, seem to explain this want of crystalli- 
zation in calcedony, by considering that substance as nothing 
more than quartz in a concrete state. This is, in fact, supposing 
it a sort of irregular crystallization, which bears the same ana- 
logy to quartz, that calcareous alabaster bears to crystallized 
carbonate of lime. I cannot, however, by any means adopt this 
opinion, as it does not appear to me to agree with those facts 
which nature permits us to observe ; for, not to mention those 
kinds of quartz which are produced by a disturbed crystallization. 
