The Scottish Naturalist. 173 



pure quartz sand as white as snow. Dr. Schleiden again 

 observes that the oil palms of the western coasts of Africa are 

 grown in moist sea-sand; and that from the year 1821 to the 

 year 1830 there were exported, as produce of these palm trees, 

 into England alone, 107,118,000 lbs. of palm oil, containing 

 76 million lbs. or 32 thousand tons of carbon — these thousands 

 of tons of carbon being furnished by trees grown in a soil that 

 was practically free from organic or carbonaceous matter of any 

 kind whatever." 



What then constitutes a soil? The answer is — any earthy 

 material whatsoever. This is putting it broadly, but it is in 

 this broad simple law that so much beneficence and wisdom is 

 seen. Had it been an essential that earth, to become a soil, 

 must contain an amount of organised carbonaceous and other 

 material, then in a world constituted as is ours no soil whatever 

 could have been formed, and carbon with every earth and 

 mineral would have retained their inorganic form. But the 

 vegetable kingdom is ever ready to transform the most arid 

 plains into beautiful gardens where physical conditions are 

 not sufficiently powerful to oppose this usurpation. 



If some extensive forests could be placed in the centre of the 

 Sahara, it would bid fair to become in time as fruitful a country 

 as any part of Africa. The shifting of the sand would be 

 checked by an increase of moisture ; and if the forests extended 

 their limits in the least degree, their conquest of the whole 

 district would become almost a certainty. In other words, if 

 perfect rest, with sufficient moisture, could be given to the 

 Sahara sands, they were then soil fit for the growth of forests 

 that would in the end produce a humus rich with vegetable 

 material. The soil of the Carse of Gowrie, and of the Tay, 

 and Earn valleys, are what I described them before as being 

 the wear and tear of the high-lying shoulders of the Grampians. 

 The floor of the Earn Valley is, as was before shown, com- 

 posed of layers of coarse pebbles, beds of sand, merging from 

 coarse material above the pebbles, to fine arenaceous, or argillo- 

 arenaceous soils at top, or in some parts from the pebble-beds 

 through sands to stiff clays at top. In all this we see nothing 

 but the re-arranged materials of older denuded rocks, and we 

 may generalize upon their origin by roughly estimating their 

 several natures. The pebble beds contain perhaps 80 per cent 

 of siliceous material : some of the pebbles are micaceous, and 

 in other ways vary from a state of pure quartz, but not to any 



