432 CHALK AND "ROTTEN LIME-STONE." 



This plan of classification has reference to the agricultural or 

 manuring characters only of the substances named. Those which 

 dounot come under the head of marl, in the extended sense adopted 

 above, and which are not important in -Virginia, will be dismissed 

 with but slight notice. 



The general and very comprehensive term calx is here used to 

 include every natural (or indeed artificial) formation of earth, stone, 

 or shells, separate or in mixture, in which carbonate of lime is a 

 considerable and the most important part. All such substances be- 

 long to one or the other of the two great divisions I., of stony hard- 

 ness, and II. , of softer or eartliy texture. 



I. The stony bodies require to be burnt to quick -lime, to be 

 used profitably as manure. Such are, 1, compact or ordinary lime- 

 stones ; 2, marl-stone, or the hardest and largest stony nodules or 

 continuous layers in softer marl ; and 3, oyster or other recent and 

 hard shells. 



II. The calcareous substances of earthy texture, soft enough to 

 be used as manures, without being reduced by calcination, all come 

 under the general and extended term marl, as here used. The 

 most important substances to be included under this head, are the 

 following : 



1, Chalk proper (nearly pure carbonate of lime), such as is 

 abundant in parts of England and France, is said by geologists not 

 to exist in North America. But there is what may be deemed, in 

 agricultural sense, an impure chalk, (2,) which spreads over or un- 

 der an immense extent of this continent. This is in Alabama and 

 Mississippi called " rotten limestone." It underlies, in beds of se- 

 veral hundred feet of thickness, large portions of these states, and of 

 Florida and Arkansas ; much of Texas, and, as I believe, most of 

 the vast prairie region between the Mississippi river and the Rocky 

 Mountains. This earth, so far as known to me by specimens only, 

 is composed of carbonate of lime principally, but with some 20 to 

 35 per cent, of clay. It is of a dingy whitish colour when dry ] has 

 about the degree of hardness of chalk, to which this earth approaches 

 more nearly in composition, texture, and colour, than to either lime- 

 stone or to true marl. It may be inferred from the words of de- 

 scription in Fremont's Report, that this is the earth which forms 

 the great region through which part of the river Platte passes, and 

 which is found from the lowest visible depths to the summits of the 

 crumbling cliffs, some of which are many hundreds of feet high, so 

 remarkable along the banks of that river. I further infer that it is 

 this chalky and highly calcareous character of the surface-soil and 

 sub-soil which renders this region generally so barren, and usually 

 so destitute of water ; while the continual crumbling of the banks of 

 the same barren earth into the river, and the earth being carried 

 down by the floods, intermixed with other suspended earths, and 



