MIOCENE MARL. 439 



farm of Dr. "William I. Dupuy, and other adjoining lands) there 

 is an exception to this general rule, the marl being found about a 

 mile farther west, overlapping the eastern and lowest part of the 

 granite, and passing under a small stream which empties into the 

 Appomattox, a mile above the lowest falls. 



The only important fertilizing ingredient of the miocene marls is 

 the carbonate of lime, or shelly matter. There may be, and proba- 

 bly is, some slight additional benefit sometimes, from accidental or 

 peculiar admixtures of other substances ; as, of animal matter still 

 remaining, or, in limited spaces, the phosphate of lime supplied by 

 bones of large fish or sea reptiles ; or of vegetable extract in blue 

 marls, of the oxide of iron, of a very small proportion of green- 

 sand generally; and even of the clay or the sand, respectively for 

 soils deficient in either. But either and all of these additional mat- 

 ters, though giving some value as manure, are of but little im- 

 portance in miocene marls, in comparison to the main and great 

 agent of fertilization, the shelly or calcareous matter. According 

 then to the greater or less proportion of this main ingredient, and 

 to its state of division or readiness to be reduced to a state of mi- 

 nute division in the soil, may be rated the comparative values of 

 marls for manure. In regard to the much larger proportions of 

 green-sand in miocene marls, as asserted by other authority, and de- 

 nied by me, some additional remarks will be hereafter submitted, in 

 the proper order for consideration. 



As might be inferred from the obvious manner of the deposition 

 of the marl, as before stated, by waters of the sea in violent and 

 yet varying degrees of motion, the different horizontal layers 'of 

 marl, successively deposited in the same bed, and even within a few 

 inches of perpendicular distance of each other, -sometimes exhibit 

 remarkable differences of appearance, composition, and of value ; 

 while there is also generally as remarkable a uniformity of charac- 

 ter of each particular layer (though differing much in thickness at 

 different places) throughout not only the different diggings of the 

 same place, but sometimes for miles in extent. I have seen often, 

 in diggings on different farms, and several miles apart, layers of 

 marl so precisely alike, and so marked in peculiar character, that 

 there could be no doubt of their being parts of the same particular 

 deposit, made at the same time, and by the same operating natural 

 causes. Under such circumstances, a practised eye can by com- 

 parison fix very nearly the chemical composition of similar varie- 

 ties, and even more correctly, for general averages of value, than 

 would be usually obtained from the accurate chemical analysis of 

 one or two specimens only. For the usual danger of error is, not 

 in the chemical analysis (which is easily enough made, and the 

 mode sufficiently correct), but in the selection of equal and fair 

 specimens of marl to exhibit the average strength of the whole body 



