EXAMINATION OP ITS WORTH. 4S9 



tity and proportion of green-sand in the marl ; and, so far as all nay 

 experience and observation enable me to judge, I cannot but believe 

 that the above stated estimates of quantities and proportions of 

 green-sand are greatly exaggerated, and extremely incorrect and 

 delusive. I do not mean to assert, and cannot be expected to prove, 

 the negative of the assertion of such abundance of green-sand. But, 

 from, all my means for arriving at conclusions, it is my confident 

 belief that but few of the bodies of miocene marls in Virginia con- 

 tain as much as 2 per cent, of green-sand if even as much as 1 per 

 cent. ; and that an average proportion, throughout any considerable 

 digging for manure, of as much as 5 per cent, of green-sand is 

 extremely rare. With but a single peculiar exception, which 

 will be described presently, the largest proportion (estimated by the 

 eye) that I ever found was supposed to be 5 per cent. ; and that 

 was in a very peculiar marl, found at Coggins Point farm and else- 

 where in that neighbourhood, or rather a loose calcareous sand, 

 which forms the overlying layer of a compact blue marl. This sand 

 contains only about 20 per cent, of finely divided shelly matter, and 

 the whole mass would appear, to slight observation, similar to, and 

 as poor and as loose, as the deep sands of the roads through a sandy 

 country. But few persons would have used this sand for manure, 

 or would have dignified it by the name of marl. However, the 

 ease with which it could be worked, and the necessity for removing 

 it to uncover the better marl below, induced me to carry out and 

 apply it as a second dressing to an adjacent part of a field which 

 had been just before marled from the richer blue layer. The effects 

 were so marked, and so superior to the single marling, that I was 

 ready to believe that the green-sand caused the difference. The 

 loose calcareous sand mentioned at page 443, which one of my 

 neighbours supposed (from its good effects) to be rich in calcareous 

 earth, is precisely like mine in general appearance, and in position 

 in the bed ; and appears to have a like unusually large proportion 

 of green-sand, which no doubt served to produce some small part 

 of the benefit which was ascribed wholly to the carbonate of lime. 

 This peculiar deposit furnishes the only cases known to me of any 

 ordinary miocene marl (if this loose sand can be so termed) being 

 rich enough in green-sand for the benefit from the latter to be 

 known. And even this benefit would not have been distinguished 

 or suspected, but that the poverty of the earth in calcareous matter 

 required it to be applied very heavily. The much thicker body of 

 compact marl, lying under this poor calcareous sand, contains (by 

 supposition) not so much as 2 per cent, of green-sand. 



But it is true, that when attention was not particularly directed 

 to green-sand, proportions not exceeding 5 or 6 per cent, might 

 have escaped the notice of one who had handled and examined 

 the specimens of marl, or who even analyzed them, merely with a 



