Action of the Atmosphere upon neioly -deepened Soil. 467 



horneblencle affords the necessary quantity of magnesia. In addi- 

 tion to these the basaltic are found to contain appreciable quan- 

 tities of phosphoric acid, so that they are in a condition to yield 

 the plant almost all its necessary constituents." {Encycl. Brit. 

 8th ed. vol. ii., Agricult. Chemistry.^ 



In Cornwall Sir H. De la Beche observed that these rocks 

 supplied the most fertile soils in the district, so much so that when 

 greatly disintegrated they are sometimes worked as marl-pits. 

 At the Lizard, in the same county, the soil on the diallage rock 

 and sienite was found to be amongst the most fertile, if not the 

 most fertile, of the lands in that quarter, exhibiting an instance of 

 the productiveness of a soil chiefly derived from the decomposition 

 of the horneblende or diallage and felspar. The horneblende 

 slate and rock at the same place is also extremely fertile. " As 

 the rocks at the Lizard," says Sir Henry, " are all exposed to the 

 same conditions, the contrast afforded by the varied fertility upon 

 these mineral compounds is highly instructive and illustrative of 

 the fact that soils do, all other things being equal, mainly depend 

 on the subjacent rocks for their agricultural character." 



The Trap hills of Skye and many of the Western Islands of 

 Scotland I liave observed to be often remarkable for the beautiful 

 green pasture which clothes their steep sides, the short sweet 

 herbage soon covering the decaying greenstone, which in many 

 places crumbles down fast under the united influences of the frost 

 and rain. Doubtless the dripping climate, raining three days out 

 of every four, as the natives say, has an effect little short of irri- 

 gation upon the pasture ; but allowing for all that, and for any 

 difference of level, the contrast between these green islands of the 

 west and the rugged heath-covered ribs of the primary and gra- 

 nitic giants of the Cairngoi'm type is too great to be accounted 

 for otherwise than by the less favourably constituted chemical 

 nature of the latter rocks. 



The solvent action of the atmosphere and rains on these mine- 

 rals is evinced by the fact of the outer portions of decayed traj) 

 having been found to contain scarcely any lime, while the central 

 kernel which that influence had not readied held a large propor- 

 tion. (Johnston, Chemistry of AyricuUure.) The same writer 

 also remarks having picked up a portion of decaying trap on the 

 farm of Swanston, on the Pentland Hills, which contained IG per 

 cent, of carbonate of lime. 



Zcolitrs. — The family of zeolites comprises a good many spe- 

 cies of minerals which agree in this res])ect, that they are com- 

 pounds of silica with generally two or more bases, and a certain 

 considerable amount of water in a crystalline state. These 

 minerals when powdered and digested with cold muriatic acid 

 are converted into a gelatinous inass. Kow as felspar, which is 



