434 SHELL SAND FRESH-WATER SHELL MARL. 



marl may be presumed to have been found and to exist in the 

 places of ancient lakes, or other still waters, in all chalk regions 

 in the vast " rotten lime-stone" and prairie region of the southern 

 and western (or interior) parts of North America rarely, if ever, 

 in our mountain lime-stone region, and certainly never in our tide- 

 water region. The calcareous beds of the tide-water region have 

 entirely a different origin, having been originally deposited or 

 formed and grown on the bottom of the ancient ocean, and since 

 upheaved to their present higher elevation. And it would be as 

 useless to search for the latter formation in the higher country. 

 Hence, the geological character of any region will indicate very 

 accurately whether either one, and which of these kinds of marl, 

 or neither of them, can be found. 



5. Sea-sand is used to great advantage as manure in some parts 

 of France, and Britain and Ireland. A large, and sometimes the 

 larger part of this sand consists of finely reduced shells, rubbed to 

 granular state by the power of the waves ; and this calcareous in- 

 gredient is the all-important fertilizing part of this manure, though 

 its operation and even its presence may be sometimes unknown to 

 the ignorant users.* 



6. Shell marl may be divided into (A) fossil fresh-water shell 

 marl, and (B) fossil sea-shell marl. 



A. The first of these kinds is what is usually, if not always, 

 understood by the name " shell marl" by English writers. It is 

 formed by the gradual accumulation of the shells of small fresh- 

 water shell-fish, of existing species, on the bottoms of the shallow 

 lakes and ponds where the animals had lived and died. When the 

 bottom had been raised by this long-continued accumulation, and 

 perhaps increased by like deposits washed from higher sources, 

 nearly to the level of the surface of the water, then water-plants" 

 began to grow and to form a new accumulation of vegetable matter, 

 intermixed with the continuing deposits of earthy matter from 

 occasional turbid floods. Finally, by these means, the lake was 

 changed to a peat-bog, wet and miry, though usually free from 

 standing water. It is usually under peat, and sometimes at con- 

 siderable depths, that this peculiar and very rich calcareous manure 

 is found. It is almost pure carbonate of lime. It has been sold 

 in Scotland by the bushel, at a high price, and in great quantity, 

 for manure. 



* A notice of the English sand, showing old opinions of its value and 

 operation, was quoted at page 381 of this Essay. 



f In the Edinburgh Farmers' Magazine, vol. iv. p. 153, there is an inte- 

 lesting article (most of which was republished in the Farmers' Register, 

 vol. i. p. 90), describing a large body of this kind of shell marl, under 

 Resteneth peat-inoss, Forfar, Scotland. Most of the shells are of the wa- 



