284 NATURAL CONDITIONS OP MARL BEDS. 



who had not experience or information of the sanitary influence of 

 calcareous manures. 



But however strong the conviction of these authors of such 

 effects of calcareous manures, they offer no satisfactory explanation 

 of the manner in which the effects are produced. But whether 

 lime, in soil, exerts its health-preserving power by " arresting the 

 noxious effluvia which tend to rise from every soil," &c. or by 

 absorbing noxious matter, or annihilating any deleterious properties 

 it possesses" or, according to my previously expressed doctrine, 

 by the power of lime (calx) to combine with the first results of 

 putrefaction, and so fix them in the soil, there to serve only as food 

 for plants the end is the same, of converting to the purpose of 

 fertilization and production what would otherwise escape into the 

 air in the form of pestilential gases. 



The important facts, recently made known by Dr. Wight, as 

 stated in a previous chapter, that the calxing of soil causes the 

 plants grown thereon to absorb from the atmosphere much in- 

 creased quantities of carbonic acid, and to evolve proportionately 

 increased quantities of oxygen gas, serve to add greatly to the be- 

 fore supposed sanitary operation of calxing land. Both the as- 

 serted actions, co-operating, are abundant and satisfactory causes for 

 the beneficial effects to health ; and of which effects there can be 

 no longer room to doubt, seeing the testimony adduced from France 

 and England, in addition to all that I had before offered. 



CHAPTER XXVI. 



THE EXCAVATION OP MARL PITS, AND CARRYING OUT AND APPLY- 

 ING OF MARL. 



THE natural features of marl beds, and their exposures, are dif- 

 ferent at almost every locality ; and therefore no one manner of 

 working will suit precisely for different diggings. Still, all the marl 

 beds of Virginia may be classed under three heads, in reference to 

 the excavation and removal of the marl. 



I. The first class is of marl exposed (or " cropping out") high 

 up on hill-sides, with but little overlying earth to remove for large 

 excavations of the marl below the marl and the adjacent ground 

 dry and free from springs and the proper sites for roads, leading 

 to the fields, either descending, or nearly level, or with not much 

 ascent. Marl so lying is often of the richest kind, containing from 

 60 to more than 80 per cent, of pure shelly matter ; and that mostly 



