NOKTH-CAROLINA GEOLOGICAL SURVEY. 95 



1st to the royal agricultural society of England, has estimated 

 the soluble phosphoric acid at eight and a half cents per 

 pound, and the insoluble at three. 



It must be recollected that in order to bring phosphoric 

 acid to a soluble condition it requires considerable expense. 

 It is better to purchase what is called the insoluble or tribasic 

 phosphates than the soluble ones which are found in our 

 markets and sold as superphosphate of lime. 



The actual value of the mineral fertilizers to farmers is a 

 question quite different from that which considers the value 

 of bone dust, or potash by the pound. Immense benefits 

 have been secured by the use of marl, which, considered in 

 a commercial point of view, was worth nothing. The phos- 

 phoric acid in a bushel of shell marl is not worth, in com- 

 merce, a penny ; but for use on worn out lands the farmer is 

 enriched more than one-fourth of a dollar after paying for the 

 labor of raising and applying it. 



We are not, however, to confine our estimates of the value 

 of a marl from its phosphoric acid and potash. Excluding 

 the sand and insoluble silica, all the soluble matters are valu- 

 able to the farmer as fertilizers, and hence the determination 

 of how much is soluble, and how much insoluble, is a more 

 correct mode of getting at the value of marl than by confin- 

 ing our estimates to the two elements referred to. 



These remarks apply only to the value of a marl for the 

 private use of an individual owner, w T ho employs his own 

 hands in raising it when there is the least to do and economises 

 his expenses to the best advantage. 



Marl, however, in its crude state, as it exists in the pits, 

 has a value which admits of estimation. The common shell 

 marl may be hauled very frequently from two to four miles, 

 and give profitable returns. This is often done. The shell 

 marl, however, will not bear transportation as far as the green 

 sand of Blackrock. 



67. I have alluded already to the difficulty of recognising 

 certain marl beds in consequence in part of the absence of 

 characters upon which geologists can rely. Among the beds of 

 which there are doubts respecting their epoch, I find a green 



