CLAY FROM TIIR FELSPAR ROCKS. 261 



fore, washed away by the first shower of rain iliat falls. The insoluble 

 silica and the silicate of alumina are either left behind or are more slow- 

 ly carried away by the rains in the form of a fine powder (a fine porce- 

 lain clay), and deposited in the valleys or borne into the rivers and lakes, 

 — while the particles of quartz and mica, having lost their cement of fel- 

 spar, fall asunder, and form a more or less siliceous sand. 



Granite soils, therefore, on all hanging grounds, — on the sides and 

 slopes of hills, that is — are poor and sandy, rarely containing a sufficient 

 admixture of clay to enable them to support crops of corn — while at the 

 bottoms of the hills, whether on flat or hollow grounds, they are com- 

 posed, in great measure, of the fine clay which has resulted from the 

 gradual decomposition of the felspar. 



This clay consists chiefly of the silicate of alumina contained natural- 

 ly in the felspar — it differs little, in short from that which has already 

 been described (p. 161), under the name of pure or pipe clay, which is 

 too stiff and intractable to be readily converted into a prolific soil. 



It will readily be understood how such soils — decomposed felspar soils 

 — must generally contain a considerable quantity of potash from the 

 presence of minute particles of silicate of potash still undecoraposed ; 

 and it will be as readily seen that they can contain little or no lime, 

 since neither in felspar nor in mica has more than a trace of this earth 

 been hitherto met with. 



We have seen, however, that hornblende contains fromi^thto |thof its 

 weight of lime, and as the same carbonic acid of the atmosphere which 

 decomposes the felspar, decomposes the silicates of the hornblende also, 

 it is clear that soils which are derived from the degradatiqsft of syenitic 

 rocks, especially if the proportion of hornblende present in them be lar^e, 

 will contain lim"^e as well as clay and silica. Thus consisting of a great- 

 er number of the elements of a fertile soil, they will be more easily 

 rendered fruitful also — must naturally be more fruitful — than those 

 which are formed from the granites, correctly so called. It is to the pre- 

 sence of this lime l^at the superior fertility of the soils derived from the 

 hornblende slates of Cornwall, already adverted to (p. 255), is mainly 

 to be ascribed. 



Schorl^ as above stated, contains much oxide of iron, and sometimes 

 five or six per cent, of magnesia. It decomposes slowly, will give the 

 soil a red colour, and though it contain only a trace of lime, yet the ad- 

 mixture of its constituents with those of the felspar may possibly amelio- 

 rate the quality of a soil formed from the decay of the felspar alone. 



It thus appears that a knowledge of the constitution of the minerals of 

 which the granites are composed, and of the proportions in which these 

 minerals are mixed together in any locality, clearly indicates what the 

 nature of the soils formed from them 7nust be — an indication which per- 

 fectly accords with observation. The same knowledge, also, showing 

 that such soils never have contained, and npver can, naturally, include 

 more than a trace of lime, will satisfy the improver, who believes the 

 presence of lime to be almost necessary in a fertile soil, as to the first 

 step to be taken in endeavouring to rescue a granitic soil from a state of 

 nature — will explain to him the reason why the use of lime and of shel. 

 sand on such soils, should so long have been practised with the best ef 



