72 
[Assembly 
notoriety, that shells are very thin, and not abundant, in waters 
containing little calcareous matter. Almost all soils contain a lit- 
tle carbonate, sulphate, or muriate of lime, some of which is car^ 
ried by the surface and spring waters into marshes, rivers, lakes, 
Mlc. The calcareous matter is there absorbed and secreted by the 
testaceous animals, so as to form an external covering for their 
protection; but in those situations where little calcareous matter 
is furnished, the shells do not accumulate so as to form shell marl, 
because the testacea of each succeeding generation, in forming 
their own shells, consume those of pre-existing ones. 
Where calcareous matter is abundant in water, testaceous ani- 
mals secrete enough from their food to form their shells, without 
the necessity of consuming the dead ones of preceding generations, 
which often accumulate in so great quantities as to form thick and 
extensive beds of marl. Shell marl is a white pulverulent sub- 
stance when dry, and when wet, is so soft that a pole may be easi- 
ly thrust into it. It is composed of the shells and decayed frag^ 
ments of the Lymnea, Physa heterostropha, Planorbis trivolvis, P. 
campanulatus, Cyclas similis, and some other species. 
Many of the beds of shell marl are covered by beds of peat from 
two to ten feet thick. Such localities were once lakes or ponds which 
have been gradually filled up with alluvial depositions, and become 
land. The marl, if present, may be found by thrusting a pole 
through the mud into the stratum below; on withdrawing it, the 
marl may be recognized as a white slimy mud on the lower part 
of the pole, more or less concealed by the black mud through 
which it has been withdrawn.* In some parts of Orange and 
Dutchess counties, this marl is much used by the farmers, and with 
great advantage to their crops. It is also used some in other coun- 
ties along the Hudson, but its value is not yet fully appreciated. 
It is desirable that agriculturists should make more extensive use 
of a manure so valuable as this, on soils that contain little lime. 
This mineral may also be used, when pure, for whiting, and for 
making lime. 
Bog Ore. 
Bog iron ore is an alluvion which is now forming. It renews it- 
self by continual depositions, in certain localities, so that the same 
* If vinegar, or any acid, be poured on this marl, it effervesces or boils up by the escape 
of carbonic acid 
