148 GEOLOGICAL SURVEY 
marl. Of this character are the beds overlying the marl, at Winnamaker’s, in Orangeburg, and at 
another locality, not far distant, on the Columbia road, At both of these places numerous casts of 
Lutraria petrosa, and other Eocene fossils, are found, but not an atom of carbonate of lime, either 
in the casts or the white beds which enclose them. 
In the village of Orangeburg there are similar beds, abounding in easts of Cardita planicosta : 
portions of these beds are rich in carbonate of lime, and others quite poor, or nearly destitute of 
lime. The removal of the lime is easily understood: nothing more is necessary, indeed, than the 
percolation of water holding in solution carbonic acid. Water thus charged, passing constantly 
through a bed of marl, would leave only such portions as were insoluble, namely the silicious mat- 
ter and alumina—precisely what is left in the beds just mentioned. The effect would, of course, 
be heightened by the presence of salts of iron, which are decomposed by carbonate of lime. 
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 or marl of the Cretaceous and Tertiary periods, it is gen- 
erally converted into lignite, but when found embedded in sand, the wood is replaced by silica, and 
assumes the form of silicified wood. 
In like manner, the shells enclosed in marl or clay are never silicified, while all the beds of buhr- 
stone, which are nothing but masses of shells, are associated with sandy strata. 
To those who have not examined the matter closely, it may appear that the lime of the shells 
was first dissolved out, and their hollow moulds filled by the infiltration of silica, held in solution. 
A little reflection, however, would satisfy any one that this was not the process; for it would be 
obviously impossible, 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 obliged, 
then, 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 taken by the silica; for in no other way 
can we account for the preservation of the most minute strie on the shells, together with every 
spine, process, or other characteristic mark. ‘The solution of the silica offers no difficulty—hot 
water will dissolve silica largely, with the help of an alkali. 
Now as these beds are along the upper verge of the Tertiary and overlap the metamorphic rocks, 
it is not a very wild supposition, that the waters of the Tertiary sea may, at one time, have been 
heated, and thus facilitated the solution of the silica. 
At an interesting locality, at Lang Syne, in Orangeburg District, to be hereafter described, I found, 
in the interior of the claw of a fossil crab, some fluid silica. At first it resembled thin, transparent 
jelly, but on exposure to the air it changed color, becoming milky, and finally became hard, and - 
presented the external appearance of pearl-stone. 
Mr. Ruffin pointed out the fact that the buhr-stone lies just at the verge of the Plearebnis beds 
of the Charleston Basin; and it is now known that no Hocene marl is found north of it. Now we 
can only account for this, by supposing that the Buhr-stone formation underlies the calcareous beds, 
and that where the latter thin out at the surface, the former makes its appearance. Had the buhr- 
stone been superimposed upon the calcareous beds, it is scarcely possible that marl would not be 
