428 CHARACTER OF TRUE MARL. 



" marl" by illiterate farmers only, has been since received under 

 that name by chemists and the scientific reporters of geological sur- 

 veys ; and thus confusion has become still " worse confounded/' 

 In the following pages, I shall be compelled, as heretofore^ to yield 

 in part to such misapplication of the term ; but at the expense of 

 some otherwise useless repetition, and frequent explanation, shall 

 hope to avoid misleading readers as to each of the particular earths 

 under consideration. And I shall in no case apply the term marl 

 to any but a calcareous earth, or mixture of earths, and of which the 

 calcareous ingredient or proportion of Carbonate of lime is deemed 

 sufficient to constitute the most important, if not indeed forming the 

 only important or appreciable agent of fertilization ; and therefore 

 I shall not so designate either the fine clays (not calcareous, or very 

 slightly so), and formerly, if not now, called marl, in England, or 

 the green-sand earths of New Jersey, Delaware or Virginia, when 

 containing very little or no carbonate of lime. 



True marl, as correctly understood by mineralogists, is a fine 

 calcareous clay, containing very little silicious sand, and none coarse 

 or separate ; of firm texture not plastic, or very adhesive ; does 

 not bend under pressure, but breaks easily, and after being dried, 

 the lumps speedily crumble when immersed in water. It is man- 

 ifest, from its laminated appearance and fracture, that this true marl 

 had been originally suspended in rapidly flowing waters, and de- 

 posited at the bottom by subsidence, when the waters became com- 

 paratively still ; as when a rapid river, turbid with calcareous clay, 

 reached a lake. Thus, from its manner of formation, such marl, 

 however argillaceous, was of a texture very different from the almost 

 pure or the most tenacious clays. The carbonate of lime also tends 

 to preserve an open and mellow texture in true marls, disposing the 

 lumps readily to yield and crumble, or fall to powder or to thin 

 flakes, under atmospherical influences, which would only affect clay 

 by making it an intractable"' sticky mortar when wet, or lumps of 

 almost stony hardness when dry. Moreover, there seems good 

 reason to believe that in true marl there Js a chemical combination 

 (and not merely a mixture) of the argillaceous and calcareous in- 

 gredients, induced by their suspension in water, when the particles 

 of both were in the finest possible state of division, and most inti- 

 mate intermixture, while so suspended. Besides the crumbling 

 quality just stated, so different from clay, there is a still stronger 

 reason for believing that the calcareous and the silicious parts of 

 true marl are chemically combined, which is, as I have found, that 

 they cannot be separated by mechanical means, such as agitation 

 and subsidence in water.* For the suggestion that the different 



*The silex and alumina which compose the purest clay, are chemically 

 combined in the proportions of nearly 65 parts of silex to 35 of alumina} 



