199 



covering the lower deposits near the water side. It would appear 

 from what we behold in these cliffs, as well as from evidence 

 collected throughout the " marl region," that the greensand con- 

 stitutes generally but a single stratum, conceived to be continuous 

 beneath the whole district, and to be itself in many places, more 

 than thirty feet in thickness. That it is occasionally divided into 

 two or even more subordinate thinner beds by bands of clay 

 and sand, which are sometimes interpolated, and swell to a 

 thickness of several feet, is obvious enough from the features seen 

 in many excavations. But it should rather be considered as a 

 deposit remarkable for the permanency of its composition, thick- 

 ness, and external features. Owing to the large amount of water 

 which it usually contains, the greensand is rarely penetrated in 

 the numerous diggings which are made in it for the marl, to a 

 greater depth than about twenty feet, the pits becoming at that 

 limit too wet to be prosecuted deeper. 



In one or two instances, wells have been sunk through the 

 stratum, and the depth of the greensand ascertained to be about 

 thirty feet, as already mentioned. 



Specific Gravity. — The specific gravity of the green granular 

 mineral, carefully freed from all extraneous adhering matter is, 

 according to several experiments cautiously made, about 2-65. 

 Three different specimens, taken from remote localities, gave for 

 the two lowest each 2-63 ; for the highest 2-70. 



The hardness of this mineral varies materially, being dependent 

 somewhat upon the time elapsed after it has been dug ; it is softest 

 when moist and recently uncovered. Freshly extracted, its hard- 

 ness often does not exceed that of talc ; but when long uncovered 

 and dry, it nearly equals that of gypsum. 



It \vould appear by experiment to be entirely insoluble in water, 

 both cold and boiling; but it dissolves with tolerable facility in 

 any of the stronger acids, though different specimens vary ma- 

 terially in this respect. 



Chemical Composition of the Green Mineral. 



From a number of analyses of the greensand, selected with the 

 greatest care for the purpose, and ascertained to be entirely free 

 from extraneous matter, it would seem that this mineral is not 



