34 Phosphate Rocks of South Carolina, 
more is necessary, indeed, than the percolation of 
water holding in solution carbonic acid.” 
‘“Water thus charged would leave only such por- 
tions as were insoluble, namely, the silicious matter 
and alumina, precisely what is left in the beds just 
mentioned.” 
“ But it is rather more difficult to account for the 
complete replacement of substances by other matter. 
When wood is enclosed in beds of clay it is generally 
converted into lignite, but when found imbedded in 
sand, the wood is replaced by silica and assumes the 
form and character of silicified wood.” In like man- 
ner shells associated with sands or sandy strata in 
Tertiary ages invariably become silicified. 
“To those who have not examined the matter 
closely,” continues Professor Tuomey, (see Geol. of 
S. C., p. 148,) “it may appear that the lime of the 
shells was, first, dzsso/ved out, and their hollow moulds 
filled by infiltration of silica held in solution. A lit- 
tle reflection, however, would satisfy any one that this 
is not the process; for it would be obviously impossi- 
ble in loose sand for any such perfect moulds to exist, 
for the moment the lime was removed, the sand would 
pour into the hollows left. Besides, the internal cast 
of the shell would fall down, there being nothing to 
support it, and the mould be spoiled. We are then 
obliged to suppose that the process went on slowly, 
and that the lime was replaced by silica, particle by 
particle; that is, that when an atom of lime was 
removed, one of silica took its place, and this was 
continued until all the lime was removed and its place 
