478 GREEN-SAND OR GYPSEOUS MARL. 



carbonate must be so changed to sulphate of lime, after any marl 

 which still contains sulphuret of iron, is applied as manure. The 

 exposure to air (and attraction of oxygen) will soon convert the 

 yet remaining sulphuret to sulphate of iron ; and this will imme- 

 diately act on the carbonate of lime, in contact, and so form sul- 

 phate of lime. This proportion of gypsum, either ready formed, or 

 soon to be formed, making altogether from 2 to 6 per cent, of the 

 marl, is one of the main sources of the early (but, as I anticipate, 

 transient) fertilizing effects of this and other varieties, which are 

 poor in calcareous matter. The long continued action of the sul- 

 phuret of iron (which seems to be still generally present, and may 

 be inferred to have been universal at first in all the beds) is suffi- 

 cient to account for the partial or total disappearance of shells, and 

 of carbonate of lime, in nearly all these layers of the one great 

 eocene bed of marl and gypseous earth, both of Pamunkey and 

 James river. 



A third variety (u) exists but in few places, and on the northern 

 side of the river. It is the highest of this whole calcareous bed 

 is dry and yellowish (being nearly or quite destitute of green-sand 

 and organic colouring matter), and though as rich in carbonate of 

 lime as the average of the whole stratum (and richer than all ?/), it 

 is much inferior in fertilizing effects, at least for some years, and as 

 long as they have been separately observed. It may be inferred 

 that this light-coloured marl is not only without the potash (which 

 green-sand contains in small proportion), but also without gypsum ; 

 and, like nearly all miocene marls, acts only by its carbonate of 

 lime. 



A fourth variety (z) is the universal thin bottom layer of this 

 calcareous stratum (and below x represented by the broad black 

 line in the figure), which forms a continuous layer of separate 

 stony lumps, like a pavement, and varying from 6 to 15 inches 

 thick. These stony masses contain 60 per cent, or more of car- 

 bonate of lime. Being difficult to dig, and to raise, this layer is 

 usually left by most marlers. On account of its greater richness, I 

 deem it the most valuable for its quantity. In a few years after 

 being ploughed under the soil, most of these lumps are softened 

 enough to crumble. 



These several layers of this one general calcareous stratum con- 

 stitute the marl mostly used in latter years, by the marling farmers 

 of this neighbourhood. My own use has embraced all these varie- 

 ties, but was mostly of the moij compact earthy marl (x), as that 

 was in greatest quantity. 



Another bed of rich calcareous marl (M,) is exposed for the few 

 miles of the most western extremity of the general eocene forma- 

 tion, in the farms of South Wales in Hanover, and North Wales 

 in Caroline county, and extending nearly to the lowest part of the 



