440 MIOCENE MARL. 



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 necessary 

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

 specimen from an equal and continuous shaving of the whole ver- 

 tical 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 more or less remote. 



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

 the animals 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 mostly 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 cal- 

 careous 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 65 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 expe- 

 rienced 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 know the marls formed by shells left in their original place, 

 by the state of the shells. Either the shells being whole, and es- 

 pecially 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 much 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 cal- 

 careous powder and clay are deposited together, with but little sili- 

 cious 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 the rich marls in and about 

 "William sburg, and in Surry, and that belt of country generally, con- 

 taining 70 to 80 per cent, of carbonate of lime. 



