VARIETIES OP MIOCENE MARL. 441 



The different varieties of miocene marls, which, will now be more 

 particularly described, are not always separated in different beds, 

 but sometimes form some of the different and even adjoining layers 

 of the same bed or digging. The differences of colour, &c., caused 

 by the greater or less quantity of various accidental ingredients, 

 however striking to the eye, are not often of much importance to 

 the value of the marl ; but only (or principally) such differences as 

 are caused by the greater or less proportion of shelly matter, and 

 its state of disintegration and division. 



Brownish yellow marl. This kind, wherever found, always 

 forms the highest layers of the particular body. That is, if there 

 be layers both of yellow and blue marl in the same body, the yel- 

 low is always above and the blue below, and never in the reverse po- 

 sition. But sometimes the yellow continues to the bottom, and 

 sometimes the blue forms the top as well as the bottom. 



Yellow marl is usually found dry ; that is, having no springs or 

 oozing waters, which are generally reached on digging lower in the 

 body. But the lower part, where wet, is sometimes, though rarely, 

 of the same yellowish or dingy white tint, so as to make it manifest 

 that the colour is not dependent on the degree of moisture or dry- 

 ness. The yellowish tint is owing to the presence of oxide of iron, 

 and is pale or deep, approaching sometimes to reddish brown, ac- 

 cording to the quantity of that colouring matter. 



Yellow sandy marl is the kind most abundant in Prince George 

 county on and at some miles distance from the banks of James river, 

 and from which some farms entirely, and others principally, in that 

 neighbourhood, have been marled. It is of shells left in their original 

 place, the filling earth being mostly of coarse sand, and the whole 

 body poor in calcareous matter, varying in its proportion usually 

 from 20 to 30 per cent, and rarely richer than 35 per cent. But it 

 is of such open and loose texture (and the more so as the sand is the 

 more abundant), that this marl is easily and cheaply worked, and 

 the labour so applied is therefore often better compensated than in 

 diggings of much richer marl. In this variety of marl, the shells 

 are usually entire, or in large fragments, but are not firm or well 

 preserved. In some beds, or thick layers, they are so finely reduced 

 that the mass seems to the eye to be wholly, as it is indeed prin- 

 cipally, a body of silicious sand. From one bed of this kind, which 

 its proprietor supposed from its appearance to be merely silicious, 

 the earth was used as sand to mix in lime-mortar for masonry, and 

 it was found to serve well for that purpose. Subsequently this 

 bed of sand was found to be enough calcareous to be used as manure ; 

 and was so used, and to such good profit, that the proprietor sup- 

 posed it to be rich marl. In that opinion, however, he was mis- 

 taken, at least as to the proportion of calcareous contents. 



Yellow clay marl. But most of the richest as well as of the 



