NOETH-OABOLDSTA GEOLOGICAL SURVEY. 109 



into the peroxide. This change is indicative of a valuable 

 marl, but it is no better subsequent to this change than be- 

 fore it. If in the greenish marl green grains can be distin- 

 guished, it may be inferred that the marl contains potash. 

 The presence of carbonate of lime, as is usually known, is 

 indicated by eifervescence when acids are poured over it, 

 and a judgment may be formed by its continuance and vio- 

 lence, whether it is rich in this substance. If it is prolonged, 

 there is a large quantity of carbonate of lime in the spe- 

 cimen under examination. So the presence of sand may be 

 detected and its quantity proximately determined by simple 

 washing. 



80. The shell marl upon the Cape Fear river belongs 

 usually to the former. A bed, however, in the bluff at 

 Brown's landing, contains the green grains alluded to, but 

 still it is readily distinguished from that upon the Tar river, 

 which is usually bluish green, and belongs to the latter va- 

 riety. 1 do not, however, attach much importance to the 

 subdivision. 



There are several beds of shell marl immediately upon the 

 banks of the Cape Fear, or within a mile of them ; and when 

 marine shells are closely packed in the strata their several 

 compositions are alike. As a representation of the compo- 

 sition of this marl, I shall select Mr. Cromarty's marl bed, 

 near Elizabeth town. It consists mainly of: 



Sand, 52.50 



Carbonate of lime, 40.25 



Peroxide of iron and alumina, 7.20 



Magnesia, , 0.75 



Potash and soda, traces. 



I have always found phosphoric acid when the peroxide of 

 iron and alumina are tested with molybdate of ammonia. It 

 is very rare for the carbonate of lime to amount to seventy- 

 five per cent. I found seventy-one per cent in Mr. Mc- 

 Daniel's marl, in Nash county. The bluish green marl of 

 Tar river is quite sandy, and yet may be regarded as a rich 

 marl. As an illustration of this fact, I subjoin an analysis of 



