31 



produced. Owing to the great abundance of shell marl in these 

 places, and the general resemblance of this material to the consti- 

 tuents of a marl bank, its probable utility in this point^of view, ap- 

 pears hitherto to have been overlooked. But regarding it in its true 

 light, as a tertiary limestone, we see no reason why it should not 

 become a source of profitable manufacture in its immediate vicinity. 



OF THE GKEENSAND, SULPHATE OF IRON, SULPHUR, AND OTHER MATTERS 

 ASSOCIATED WITH THE MARL BEDS. 



Greensand. — As already intimated, this substance is frequently 

 found disseminated in the marl, and also in the overlying stratum of 

 clay or sand. From the remarkable effects of comparatively small 

 quantities of this material when applied to land, there can be no doubt 

 that many of the marls of lower Virginia owe some of their value 

 to its presence. Supposing only as much as ten per cent, of this 

 substance in a marl, and this is far below the amount which I have 

 ascertained to exist in many localities, one hundred loads of marl 

 would correspond to ten of the greensand, an amount which in New 

 Jersey has often been found productive of striking benefit. Several 

 of the most efficient marls which I have examined, were more 

 remarkable for the large proportion of this substance contained in 

 them than for their richness in calcareous matter. In many marl 

 pits which I have visited, the impressions of the pick and spade were 

 streaked with green marks, which upon inspection were found to 

 result from the bruised granules of this matter. In such cases, there 

 can be no doubt of the existence in the marl of an amount of green- 

 sand capable of affording material aid to the growing vegetable. In 

 the layer immediately above the marl, also, it sometimes exists in 

 considerable quantity — and hence instead of rejecting this overlying 

 mass, in many cases it would be decidedly better to carry it out 

 upon the land along with the calcareous matter. The experience of 

 many farmers has already shown the propriety of this plan, and 

 some even entertain the opinion that this upper layer, where the 

 greensand abounds, is quite as beneficial as the marl itself. Fur- 

 ther observations respecting the greensand will be given in treating 

 of the Eocene marls, of which it constitutes a very important ingre- 

 dient. 



Sulphate of Iron and Sulphur. — In some parts of the Miocene 

 district, there occur beds of clay more or less sandy, and usually of 

 a dark colour, containing these substances in a minute but still ap- 



