200 CALCAREOUS MANURES— APPENDIX. 



(iegrees of motion, the different iiorizontal layers of marl, successively de- 

 posited in tlie same bed, and even witliin a few inches of perpendicular 

 distance of each other, sometimes exhibit remarkable differences of ap- 

 pearance, composition, and of value; while there is also generally as re- 

 markable a uniformity of character of each particular layer, (though differ- 

 ing much in thickness at different places) throughout not only the different 

 diggings of the same place, but sometimes for miles in extent. I have 

 seen often, in diggings on different farms, and several miles apart, layers of 

 marl so precisely alike, and so marked in peculiar character, that there could 

 be no doubt of their being parts of the same deposite, made at the same 

 time, and by the same operating natural causes. Under such circumstances, 

 a practised eye can hy comparison fix very nearly the chemical com- 

 position of similar varieties, and even more correctly, for general averages 

 of value, than would be usually obtained from the accu) ate chemical analy- 

 sis of one or two specimens For the usual danger of error is, not in the 

 chemical analysis, (which is easy enough made, and the mode very cor- 

 rect,) but in the selection of equal and fair specimens of marl to exhibit the 

 average strength of the whole body excavated ; which requires much more 

 experience and accuracy than are usually exercised by most operators, 

 and still more in regard to proprietors who send specimens of their marls 

 to be analyzed by other persons. It is highly important to the farmer to 

 know the strength of the marl he is using. And to this end, it is neces- 

 sary that every layer should be carefully analyzed, or what is better, 

 a specimen from an equal and continuous shaving of the whole vertical section 

 of a digging, so as to furnish a fair average of the whole body. But after 

 this trouble is once taken, the general result will serve for all the future 

 diggings at the same place, and also for similar bodies niore or less remote. 

 The layers of marls formed by shells left " in place," or where the ani- 

 mals died, are in general the poorest; and for this obvious reason, that all 

 the hollows of and interstices between the shells are filled by what is most- 

 ly earth, (but mixed with more or less of shelly fragments,) and that earth 

 is principally silicious sand. Marl so formed, will not have more than 35 

 to at most 40 per cent of calcareous matter, and more often only from 25 

 to 35. The sand or earth that would be required to fill all the hollows and 

 chinks of a body of entire shells, of ordinary form, though touching each 

 other at their edges and points, would necessarily be as much as G5 to 75 

 per cent, of the whole mass. And therefore, it is only because of, and in 

 proportion to, the quantity of shelly particles mixed and borne along with 

 the earth brought by currents and deposited among the whole shells, that 

 such marl is sometimes richer than 25 to 35 per cent, in calcareous matter. 

 The degree of admixture of shelly fragments in this filling earth, may be 

 easily judged of by an experienced eye, and the proportion of shells and large 

 fragments will depend much on the forms of the prevailing kinds of shells. 

 It is easy to knov/ the marls formed by shells left in their original place, 

 by the state of the shells. Either the shells being whole, and especially the 

 more fragile varieties, or the two sides of bivalve shells being found in 

 close contact, as when the animal was living, will show clearly that the 

 dead shells had not been agitated or borne along by currents. The beds 

 or layers formed by removal are as easily known by the broken and finely 

 reduced state of the shells. These marls are usually much the richest in 

 calcareous matter ; for, by the grinding operation of the currents, and the 

 difference of specific gravity in the particles carried along, the calcareous 

 powder and clay are deposited together, with but little silicious sand. 

 Among the richest marls are some having whole shells in their original places, 

 but of which the interstices are filled by such fine calcareous and clayey 

 earth as could have been deposited only in waters nearly still. Such are 



