CALCAREOUS TUFA. 



325 



mechanically on the soil, and renders the clay or sand with which it 

 is intermixed, better suited to the proper expansion of the roots, and 

 more disposed to modify the power of retaining or absorbing the re- 

 quisite degree of heat and moisture, which particular vegetables may 

 demand. 



Where earths are properly intermixed, instances are known of land 

 producing a succession of good crops for many years, without fallow- 

 ing or manure. On the summit of Breedon Hill, in Leicestershire, 

 I have seen a luxuriant crop of barley growing on land, that had 

 borne a succession of twenty preceding crops without manuring. 

 This is more deserving notice, being in an exposed and elevated 

 situation, and upon the very hill of magnesian limestone, which has 

 been so frequently referred to by chemical writers, as peculiarly un- 

 favourable to vegetation. The limestone of this hill contains above 

 20 per cent, of magnesia.* 



The temperature requisite for the growth of plants is influenced 

 by the power of different soils to absorb and retain heat from the 

 solar rays, which depends much on their moisture and tenacity. 

 "It is a well known fact, that the vegetation of perennial grasses in 

 the spring, is at least a fortnight sooner, on limestone and sandy soils, 

 if not extremely barren, than on clayey or even in deep rich soils : 

 it is equally true, but perhaps not so well known, that the difference 

 is more than reversed in the autumn." — Observations on Mildew, by 

 J. Egremont, Esq. This effect Mr. E. ascribes, with much proba- 

 bility, to the rich or clayey soils absorbing heat slowly, and parting 

 with it again, more reluctantly than the calcareous soils, owing to the 

 greater quantity of moisture in the clay, which is an imperfect con- 

 ductor of heat. 



Calcareous soils might frequently be much improved by a mixture 

 of clay, sand, or gravel, which, in many situations, is practicable with 

 little expense, and would well reward the labour of the experimental 

 agriculturist. 



Calcareous Tufa, — Beside the new land formed by alluvial depo- 

 sitions, beds of calcareous tufa are sometimes formed in valleys, and 

 at the bottom of lakes, by a process which bears some analogy to 

 chemical formations. Springs that issue from limestone strata, and 

 therefore contain carbonic acid, often hold particles of carbonate of 

 lime chemically dissolved in the water ; but, on exposure to air and 

 light, the carbonic acid, having but a slight affinity for the particles of 

 limestone, evaporates, and leaves them to precipitate and form "cal- 

 careous incrustations : these, in a course of years, form thick beds, 

 and are, sometimes, sufficiently hard to be used for building-stone. 



* The magnesian lime acts, more powerfully, in destroying undecomposed ve- 

 getable matter than common lime, and its effects on land are more durable : hence 

 it is in reality of greater value in agriculture, as a much smaller quantity will an- 

 swer the same purpose. 



