GREEN-SAND OR GYPSEOUS MARL. 477 



figure, except the Hack line at bottom), is the richest in calcareous 

 matter, and much the best as manure. This is mostly of compact 

 and uniform earthy texture and appearance of dark gray colour, 

 with a greenish tint in some cases. The shelly matter, for the 

 greater part, is finely reduced, the fragments being generally so 

 small as not to be obvious to the sight. But few shells, mostly of 

 the harder gray kinds, remain entire ; and of these, the saddle oyster 

 furnishes nearly all of the perfect and still very hard specimens. 

 Near to the bottom of this layer the marl is somewhat softer and 

 poorer, and yet the entire though very soft shells are there nu- 

 merous. This marl (x) contains from 35 to more than 40 per cent, 

 of carbonate of lime, on an average. 



Above this richer part (x), the marl (marked 5 in the figure) is 

 softer, and in some degree admits the slow penetration of water, to 

 which the other marl (x) is a perfect barrier. In other respects 

 this (y) appears to the eye very similar, and not less rich in calca- 

 reous matter than that below. But in fact it does not contain more 

 than proportions varying from 30 to as little as 11 per cent., and 

 usually becoming poorer as nearer to the top of this layer. This 

 marl (?/), more generally than the richer below, I have found to 

 contain finely divided sulphuret (or bi-sulphuret) of iron, as does 

 the gypseous earth of James river, and also the gypseous earth of 

 Pamunkey. This combination of sulphur and iron, when exposed 

 to air, changes gradually to sulphate of iron (copperas) ; and this 

 last, and the carbonate of lime of the marl, decompose each other, 

 and one of the new products is sulphate of lime (gypsum), in place 

 of proportional quantities of the decomposed copperas and shelly 

 matter. This process has doubtless been proceeding for ages in the 

 bed, though very slowly for want of air ; and has served to remove 

 much of the formerly existing shelly matter which was first thus 

 changed to sulphate of lime, and this soluble substance was then 

 mostly carried off by the slowly percolating water. This decom- 

 position and subsequent removal of the lime also served to make 

 pervious the before compact and impervious marl, and thus per- 

 mitted more easily the progress of further decomposition and re- 

 moval of the former calcareous portion. However much of the 

 produced gypsum has been thus slowly dissolved and removed, there 

 is still a considerable proportion remaining. Thus, this part (?/), 

 especially, not only contains a notable proportion of gypsum before 

 formed by this process, and not yet removed in solution by the 

 slowly percolating water but there is also generally present (in y 

 especially), more of material, in the as yet undecomposed sulphuret, 

 to form more gypsum hereafter. I infer that this mode of conver- 

 sion of part of the carbonate to sulphate of lime has served to more 

 or less diminish, and in some layers to remove entirely, the consi- 

 derable amount of carbonate of lime formerly contained. More 



