FORMATION OF NATUBAL SOILS. 17 



37. Such is a brief sketch of the geological condition of the British 



islands at the beginning of the tertiary formation ; for the picturesque dales, 

 and beautiful valleys, and rolling and undulating plains, which give beauty 

 and variety to the country, were as yet at the bottom of the ocean, which 

 swept the base of the mountain-ranges or lacustrine beds confined to the 

 bosom of the mountains from which they had not as yet forced a passage. 

 Undulating plains of verdure have succeeded to the ocean-wave. The rugged 

 rocks of secondary formation, which now give character to the landscape, were 

 still washed by it. 



38. The agencies which have operated in the greatest degree in producing 

 these changes, are chemical and mechanical. The great chemical agent is at- 

 mospheric oxygen, for which many of the mineral elements possess a powerful 

 aflBnity. Rocks have been broken up, and whole masses of them have crumbled 

 into small fragments, which, by means of further accessions of oxygen, have 

 finally crumbled into dust. 



39. Again, carbonic acid, contained in great abundance in rain-water, has a 

 powerful influence in dissolving the carbonate of lime present in limestone 

 rocks : where a considerable proportion of clay is present, it crumbles into 

 powder, furnishing what is known as marly soil. Even on felspars, granite, and 

 other crystalline minerals, water exercises a highly-important action, decom- 

 posing them into alkahne silicates, yielding, in their turn, silica and carbonate 

 of potash, and silicate of alumina, — the chief constituents of clay ; in other 

 words, clay is produced; and highly-fertilizing alkahne salts, which exist 

 in these minerals, are changed into easily-soluble carbonates, which are thus 

 rendered available for the immediate food of plants. 



40. Plants and animals also take an active part in this disintegration of 

 rocks. In the celebrated experiment of Von Helraont he planted a willow- 

 tree weighing five pounds, in two hundred pounds of eai-th previously dried in 

 an oven. After an interval of five years he pulled up the willow and found 

 that its weight had increased to a hundred and sixty-three pounds three ounces. 

 During the five years the earth had been duly watered with rain-water ; but in 

 order to protect it from any foreign admixture of soil, a piece of tin plate was 

 laid on its surface, pierced with small holes ; the leaves, which fell annually, 

 were not included in the weight. The earth was oven-dried and weighed, and 

 was found to have lost only about two ounces of its original weight. Thus, 

 according to Von Helmont, a hundred and sixty-three pounds of wood, an(? 

 all the leaves of five years' growth, had been produced from water alone. Vou 

 Helmont's idea was that water generated earth, which is altogether fallacious ; 

 but the experiment is a valuable illustration of the power which vegetables 

 have of decomposing carbonic acid and absorbing carbon. Subsequent ex- 

 periments demonstrate the possibility of certain vegetables deriving the whole of 

 the cai-bon necessary to their existence from water alone.* Besides this, seeds 

 of lichens and mosses floating in the air at last attach themselves to the 



• " Handy-book of the Chemistry of Soils." 

 C 



