IV. 
EGYPT. 163 
and some chalcedonies containing almost as much chap. 
alumine as the Egyptian jasper, when thus ex- 
posed to the continued action of air and 
moisture, gradually decompose, and assume 
the white colour common to the matter of sileoc 
when in a state of extreme division. But these 
pebbles, although constantly exposed to the 
nightly dews of a country where water falls 
during the night as abundantly as heavy rain, 
and to the powerful rays of a burning sun 
during the day, have sustained little or no altera- 
tion. They have also another very remarkable 
character. Although they be destitute of that 
whitish surface which is common to every sili- 
ceous body long acted upon by the atmosphere, 
they are always characterized by a lighter 
colour towards the center of each pebble ; and 
this is sometimes white. They vary in their size, 
from that of a hen's e,^^ to the e,^^ of an ostrich; 
but are rarely larger, and always appear more 
or less flattened, so as to exhibit a superior and 
an inferior elliptical surface upon each specimen. 
The masses of mineralized or petrified ivnod had 
no regularity of shape, except that parasitical 
form which the mineral, thus modified, had 
derived from the vegetable whose fibres it had 
penetrated when in a fluid state. It is evident, 
therefore, that these pebbles do not owe their 
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