442 VARIETIES. 



poorest miocene marls, are yellowish. When rich, say containing 

 proportions of carbonate of lime from 45 to 80 per cent., the marl 

 is usually formed of shells broken down, when under the sea, to 

 small fragments or to powder, by the grinding action of the water 

 in violent motion, and left afterwards to settle in stiller water, ac- 

 cording to the specific gravity. Or it is the same kind of rich and 

 finely divided water-borne matter deposited on and filling the hol- 

 lows in and between whole shells remaining in their V>riginal 

 place. In either case, the small quantity of earth first suspended 

 in the current, and then deposited with the finely reduced shelly 

 matter, is mostly if not entirely clay ; as silicious sand, having more 

 specific weight, could not be suspended by the current so long, or 

 carried so far, before being deposited. The few rich clay marls of 

 Prince G-eorge are of the first-named variety, or composed entirely 

 of fine fragments of shells intermixed with clay. The much richer 

 marls in and about Williamsburg are of the other kind, there being 

 also numerous whole shells in place, as well as the interstices being 

 filled almost entirely by water-borne fragments, and fine powder of 

 other shells. The other contents, making from 15 to 25 per cent. 

 of the body, are principally of a very fine clay of pale yellow, and 

 much less of silicious or white quartz sand, oxide of iron, and a 

 little green-sand. Much of the same kind of rich marl is also in 

 other parts of James City and York, in the lower part of Surry, 

 and in Isle of "Wight, New Kent, and King William counties, which 

 I have seen and probably throughout the middle belt of the marl 

 region of Virginia. There has been little or none of this rich clay 

 marl seen by me in the upper range of marl counties (those next 

 the falls of the rivers), and not much more near to the eastern 

 limits, or next to where the marl dips so deeply, as to disappear 

 from the surface, and is accessible only by deep digging. Perhaps 

 observations more extended than mine have been, might present 

 different conclusions. 



The rich marls just described, when separated mechanically (by 

 the sieve, and by carefully washing in water), seem to consist, for 

 the much greater part, of pure shelly matter, mostly in large or 

 small fragments, slightly coloured brown by oxide of iron, and the 

 remainder of a very fine and apparently pure pale yellow clay. But 

 this clay is also composed in part of finely divided carbonate of 

 lime; and the fine shelly matter is intermixed with some silicious 

 sand and a little green-sand. The bed of marl near Surry Court 

 House (which is- similar to the marl at most other places thereabout) 

 is of this kind and general character ; and from it, a large body of 

 land has been manured with great benefit. This body of marl was 

 reputed, upon the authority of the State Geological Surveyor, to be 

 among the richest in green-sand. From a much larger sample of 

 the marl of this bed ; carefully selected by the proprietor, at rny re- 



