And the “ Great Carolina Marl Bed.” 33 
Lexington, is only exposed when the overlying or 
superimposed sands are removed by the winds, rains, 
or other denuding geological agents; and whenever 
thus exposed they are found to be “ BUHRSTONE”’ or 
“ VIILLSTONE-ROCK,” or beds of ‘Si~iclous SHELLS.” 
The Marl beds are sometimes interstratified with clay, 
and then the shells only are silicified, and the clay 
retains all its normal characteristics. 
When the shells are imbedded in a mass of shelly 
matter, like that of the Santee Marls, and have been 
covered by beds of sand, and been subjected to 
the chemical influence of these sands, the whole mass 
is converted into a “ Buhrstone,’ having not a par- 
ticle of its original lime remaining. But the reader 
will ask, How is this accomplished ; give us the pro- 
cess in detail? Well, we will try and explain it. 
The silica is obtained from the sand beds above. 
“Hot water will dissolve silica largely, with the 
help of an alkali,” (remarks Professor Tuomey,) “and 
as these beds overlap the igneous rocks, it is not a 
wild supposition that the waters of the Tertiary sea 
may have been at one time heated, and thus facilitated 
the solution of the silica.” 
Besides, it cannot be denied, that before the 
upheaval of the coast beds, the waters of the Atlantic, 
(the warm waters of the Gulf Stream,) passed over 
the Eocene beds. Thus we see how the silica was 
furnished from the sand hills. “ But the removal of 
the lime, of which the shells are mainly composed, 
must first take place before the silica could be substi- 
tuted, and this also is easily understood. Nothing 
2? 
2, 
