COMPARATIVE VALUES OP MIOCENE MARLS. 449 



of shells, though no shelly nor even any calcareous matter now 

 remains. In other marls, there is often seen an upper layer 

 coloured brown by this deposit of iron. Both these are different 

 modes of the same operation ; the waters charged with sulphate of 

 iron having in the latter case decomposed and removed but part, 

 and in the former all the calcareous matter, to some depth below 

 the former top of the stratum of marl. The marl, in the upper 

 part of which the shells have been thus dissolved and removed, 

 has a decided sulphureous odour, which is left very perceptible on 

 the hands, after handling the marl as dug ; and this odour is still 

 more manifest in the marl when it has been dug and thrown out, 

 and exposed some days to the .weather. Such marl is within a few 

 miles of Richmond, at Dr. Chainberlayne's and Col. C. W. Gooch's 

 farms. It is poor in calcareous matter. 



The comparative values of marls are fixed by the comparative 

 proportions of carbonate of lime contained, other circumstances 

 being alike ; yet if these other circumstances are very different, 

 they may make a marl containing but 25 per cent, worth more 

 than another of 50 per cent. The more finely reduced, or the more 

 soft the shells, the quicker the action will be, and the more profita- 

 ble the marling. But all the white shells, however hard and 

 entire when applied, are dissolved in a few years, if the soil really 

 needs so much lime that is (according to my views), if there be 

 acid of soil enough to combine with the lime. But the gray or 

 slate-coloured shells seem to be insoluble and almost indestructible, 

 and do very little good as manure. These shells are the several 

 species of scallop (pecteii) and of fossil oyster (ostrea), and some 

 few others, all fortunately being but in small proportion compared 

 to the numerous white and softer shells. Some beds of marl, 

 however, or layers, have mostly these hard shells, and therefore 

 are worth very little compared to what their chemical analysis 

 would indicate. 



It is not necessary to speak otherwise than very concisely as to 

 the practical applications and effects of miocene shell marl ; for 

 this is the kind in general use throughout lower Virginia and Mary- 

 land, and to such small extent as has been used in North Carolina, 

 and. therefore the operation is well known. All the usual and 

 general and highly beneficial effects of marl known, with but few 

 exceptions in the limited districts of eocene marl (hereafter to be 

 described), are due to the miocene marls. And of such effects 

 there have been numerous statements, general and particular. The 

 operation of the eocene marls, and especially those largely mixed 

 with " green-sand/ ' is different, and superior; but their use has 

 been so limited, and so few statements of effects published, that 

 nearly all the particular results and general statements of effects 

 38* 



