NORTH-CAROLINA GEOLOGICAL SURVEY. 



111 



sand being coarse, presents a favorable condition for effect- 

 ing a separation. 



81. The green shell marl of Mr. Bridger's plantation, 

 upon Fishing creek, I found to possess a composition similar 

 to Col. Clark's. There is a greater proportion of sand, but 

 the available part is almost identical with the Tar river marl. 



82. The application of marl is an important matter, and 

 requires a brief discussion. Notwithstanding marl has been 

 used for many years, still there is much disagreement among 

 planters of experience as to the best mode of applying it, and 

 the quantity to be applied in any given case. Its effects are 

 frequently deleterious if a large quantity is spread upon a 

 poor soil, and yet it has not been ascertained how its injurious 

 effects may be obviated. It is no doubt desirable in many 

 instances to use a larger quantity of marl than the soil will 

 admit of when it is in its natural state. 



The quantity of marl which is usually spread upon an acre 

 of ground is from 150 to 200 bushels. Three hundred bushels 

 is often used. But certain worn out lands would be exceed- 

 ingly injured for several years by even two hundred bushels. 

 The question, I have no doubt, has been often put: Why is 

 marl ever injurious? The natural conclusion is that it con- 

 tains some substance unfriendly to vegetation. This substance 

 is no doubt in certain cases an astringent salt, formed in those 

 marls which contain iron pyrites which is prone to decompose 

 on exposure to those bodies which contain oxygen, the sul- 

 phur thereby is oxidated, and slowly acts upon the iron and 

 forms copperas, or upon alumina, which is present in the marl. 

 In small doses copperas will not fatally injure vegetation, but 

 operates beneficially. The term in common use for express- 

 ing the effect of injurious marls is, burning. Those which 

 are decidedly burning marls have the distinct taste of cop- 

 peras, sometimes it appears upon the surface of those marls 

 in dry weather, when it has a whitish appearance. But 

 gypsum sometimes appears also. This may be distinguished 

 from copperas by being tasteless. 



83. There is no difficulty in treating marls in which cop- 

 peras is found. It is readily decomposed by lime. Let a 



