the moisture from the chalk, but it can alfo be carried home in fummer, when the weather and roads are at the best; and a much fmaller quantity will produce an equal effect, than when it is in the ftate of chalk.

Thofe, therefore, who have no other calcareous manure within reach of them but chalk,-when that is at a confiderable diftance, ought always to drive it in the state of lime. But thofe who are close by the pit will, in general, find it more oeconomical to employ it in the state of chalk.

$28.

Chalk fo much abounds in the fouthern parts of Britain, that fhips fometimes bring it as ballaft to the north; on which occafions it may be purchased at a moderate price by the farmer. But, although it contains, perhaps, nearly an equal quantity of calcareous matter as the fame bulk of fome very

pure

pure kinds of lime, yet it will not be good oeconomy in him to purchase it at the fame

price with the lime,—as at least three or four times more chalk than lime will need to be applied to his foil before it produces an equal effect. For, as it is impoffible to get that hard dry chalk reduced to fmall enough parts, a great quantity must be applied before it can produce any fenfible effet; and, although the effects of this manure may be lafting, yet it is never any thing nearly equal to lime, if applied in equal quantities.

Another calcareous matter of great utility as a manure is marle; the diftinctive perties of which fall now to be confidered.

$29.

OF MARLE.

pro

Few fubftances appear under a greater diverfity of forms than marle. Hence it is u

fual

fual for writers on agriculture to enumerate as diftinct manures the feveral varieties of this general class of bodies. But, as all the different kinds of marle that have hitherto been discovered may be reduced to two general claffes, viz. earthy marles, which are always found in foffil ftrata under the earth, and Shell marle, which always retains evident marks of its animal origin, I fhall confider each of these separately, as diftinct fubftances.

$ 30.

Of Earthy or Fofil Marle.

The varieties of this clafs of bodies are diftinguished by names, fuggefted by the appearance they affume when fresh dug from their native beds. When they are foft and of an uniform texture, they are called clay marles;-when firm and hard, stone marles; -when these affure a thin foliacious appearance, they are denominated flate marles, and so on.

But,

But, whatever appearance they affume when fresh dug, or by whatever name they are known, they all agree in this, that, if they be exposed for a fufficient time to the action of the air, they crumble into smaller parts, and fertilize the earth to which they have been properly applied.

The ingenious Dr Ainflie has demonftrated, by an accurate fet of experiments, recorded in the Physical and literary effays, volume third, That all the varieties of this class of bodies contain a confiderable proportion of clay, united with calcareous matter; whereas limeftone, if it does not confist of pure calcareous matter, is usually united with fand in various proportions.

The calcareous matter in marle does not differ in any respect from that in limestone, and its proportions in many cases is the fame in marle as in limestone,-so that the difference between the appearance and qualities of these two substances arises intirely from

the

the nature of the heterogeneous bodies mixed with the calcareous matter.

When marle is expofed to the air, the clay in its compofition abforbs the moisture that falls from the clouds,-fwells with it,becomes soft, and, gradually lofing its cohefion, crumbles to pieces.-If limestone is exposed to the air, the fand in its compofition is not in the least affected by moisture, and it retains its original figure and dimenfions for a great length of time.

When fand is mixed with the clay that enters into the compofition of marle, it affumes a ftony like appearance and is more or less firm according to the quantity of fand, or other circumftances.-But, where there is clay at all in the compofition, it will be gradually foftened by water acting upon it; and, it is owing to this circumftance alone, that ftone-marles fall in time to pieces when expofed to the air.

But,