20 



♦ KNOAVLEDGE ♦ 



[Jolt 13, 1883. 



attributable to the fact that they are watered by streams 

 from the higlily calcareous rocks of the Cotteswolds. Per- 

 haps in no county in England is irrigation less practised 

 than in ^\'ar^vickshire — a fact wliieh is easily explained by 

 the physical features of the district, for the majority of its 

 streams originate in the elevated plateau around Birming- 

 ham, known as the ^vaterslied of England, and have not yet 

 been long enough in contact with the rocks to have ac- 

 quired enough dissolved mineral matter to make them valu- 

 able as fertilizing agents. Just as a fertile oasis marks the 

 position of a spring of water in the parched and barren 

 desert, so, though in a less degree, is the fertility of our 

 English soils influenced by the peculiar character of the 

 water of each geological formation. The saliferous marls 

 of Staffordshire give rise to such brackish streams, that on 

 their banks sea-side plants are frequently to be found ; but 

 the injurious influence on agriculture of an excessive 

 amount of soluble matter in the underlying rocks is best 

 seen in the immense natron plains of Hungary, between 

 Arad and Debriczin, which are rendered unproductive by 

 an efflorescence of soda-salts from the soil ; while in some 

 low tracts of land on the shores of the Mediterranean, 

 •which are too salt for cultivation, it has been found neces- 

 sary to remove the saline matter by artificially flooding 

 with fresh water, and afterwards draining. 



Perhaps no geological agency exerts so much influence 

 upon agriculture as denudation — that slow and silent action 

 whereby the surface of the earth has been moulded into its 

 present form. Thus the mechanical action of running 

 water acquires an agricultural interest almost as great as 

 its chemical influence, which we have just considered. Not 

 only are immense quantities of the most productive .soils 

 carried away by this means, but by being deposited in 

 other localities, the most fertile admixtures are often pro- 

 duced. This accounts for the great improvement which 

 soils usually exhibit near the outcrop of other formations. 

 The peculiar fertility of the hop district of Farnham is due, 

 to a great extent, to the many outcrops, in a small area, of 

 beds of different character, and to the consequent mixing of 

 different soils. The great value of alluvial soils, also, is to 

 be explained V>y this natural admixture of sediment carried 

 down by running water from strata varying in mineral 

 composition. The Golden valley of Herefordshire owes its 

 superiority to a fertile alluvium derived from the corn- 

 stones of the surrounding Old Red Sandstone formation. 

 So valuable, indeed, is the detritus carried down into the 

 sea by rivers, that, in favourable localities, the flood-tide is 

 made, Ijy artificial means, to overflow the fields around and 

 to deposit its precious freight of fertile mud upon the 

 farmer's land. The inundations of the Nile are beneficial 

 to the soils of Egypt only in proportion to the quantity of 

 finely-divided sediment which is thus deposited upon them, 

 and Duponchel has even conceived the idea of fertilising 

 the barren lancles of Gascony by artificial warping from the 

 Pyrenees. This artificial admixture has lieen practised 

 with great success in many parts of England, and many a 

 barren soil has been rendered productive by the processes 

 commonly known as claying, marling, and chalking. By 

 judicious admixture of materials from neighbouring strata, 

 many of the waste lands of England have been permanently 

 improved, and have increased in value both to the owner 

 and tenant. 



But if advantage is gained by the farmer who cultivates 

 the rich alluvial tracts, the fertility of which may be traced 

 to the nature of, perhaps, distant strata, it is far other- 

 wise with those from who.je land the sediment has been 

 transpoi-ted. In many districts agricultural operations are 

 seriously impeded by the rapidity with which the soil is re- 

 moved l>y denudation. This is nowhere so well shown as 



on steep slopes, such as the sides of the granite hills of 

 Cornwall, or in incoherent soils like the Black Earth of 

 Russia, in which huge ravines are formed with surprising 

 rapidity, and the most fertile alluvium in Europe quickly 

 swept away. 



The presence of igneous dykes seems invariablj' to exert 

 a beneficial influence upon the soil. In some parts of 

 Cornwall, where the general rental of the land is about 

 thirty shillings an acre per annum, the accidental occur- 

 rence of an igneous dyke improves the land to such an 

 extent that the produce is nearly doubled, and the rental 

 consequently increased. In a similar way the Carboniferous 

 limestone soils of Derbyshire are greatly impro\'ed by the 

 presence of those volcanic rocks which are locally known as 

 toadstoac.s, and in some parts of South Wales, where trap- 

 rocks occur, an otherwise barren soil is completely changed 

 and rendered capable of producing nearly every crop. It 

 is interesting to note how quickly the lavas and ashes of 

 Etna are transformed into vineyards or gardens by the 

 ceaseless industry of the husbandmen, even although they 

 are repeatedly destroyed by fresh eruptions. 



It generally happens that in those localities where the 

 nature of the rocks has resulted in the production of the 

 most pleasing scenery, there agriculture is least developed. 

 Of course, where the scenery is bold and mountainous, we 

 shall expect to find great hindrances to agricultural opera- 

 tions ; but even in districts such as the Isle of Wight or 

 Surrey, we find a pleasing landscape accompanied by a most 

 backward state of agriculture. Nor does it often occur 

 that great agricultural value characterises those districts 

 which are famous for their mineral wealth. The mining 

 districts of England are chiefly noticeable for the general 

 unproductiveness of the soil, which is sometimes so poor 

 that three or four shillings an acre is the utmost rental 

 that can be obtained. 



Enough has now been said to show the general con- 

 nection between agriculture and geology. In the New 

 World, where there is no lack of available gi-ound, colonists 

 need cultivate only the most favourable districts — the 

 allu's-ial plains and valleys watered by running streams ; 

 but the crowded population of Europe necessitates a con- 

 tinual struggle against natural agencies to improve every 

 available spot. Thus by ceaseless toil and industry the 

 greatest obstacles have been overcome. The natural aridity 

 of Egypt, the deficient water-supply of Lombardy and 

 Tuscany, are conquered by the most careful irrigation ; 

 wastes have been reclaimed ; marshy tracts have been 

 rendered fertile by draining ; the hills on the banks of the 

 Moselle and the Rhine and the steep slopes of Provence, 

 in spite of the rapidity with which the soil is washed away, 

 are kept clothed with vines, corn, and olives by the endless 

 toil of terracing the hill-sides, and thus overcoming the 

 obstacles which nature has opposed to successful cultivation. 



The Highest Bridge in the World. — This is the 

 railway viaduct of Garabit, in France, now being erected 

 over a river in the department of Cantal. The total 

 length is about 1,880 ft, and near the middle of the great 

 centre arch, the height from the bed of the river to the 

 rail is 413 ft. The viaduct was begun in 1881, and is to 

 be completed next year. 



The longest bridge in the worM is said to be in China, 

 at Sangang, over an arm of the Chinese Sea. It is five 

 miles long : the roadway is 70 ft. high, and stands upon 

 300 arches. The parapet is a balustrade, and each of the 

 pillars, which are 75 ft. apart, supports a pedestal on 

 which is placed a lion 21 ft. long, made of one block of 

 marble. 



