GEOLOGY AND HISTORY. 503 



ter, Lewes, Reading, Wallingford, Cambridge, and Beverley, were all 

 places of great mediaeval importance, and all stand within the creta- 

 ceous area. Other wool-growing tracts of course possessed a similar 

 value. 



A few more special agricultural features of the various secondary 

 or tertiary geological formations may here he fitly introduced. The 

 Trias and other " Poikilitic " strata, running across England from the 

 Tyne to the Exe, form beautiful undulating country, comprising much 

 of the best wheat-growing and pasture land, and famous for the pro- 

 duction of cheese. In this belt lie the vale of York, the Trent and 

 Severn Valleys, the Cheshire Plain, and the vales of Exeter and Taun- 

 ton. An outlier forms the valley of the Eden at Carlisle. The Lias, 

 which follows the Poikilitic series to the southeast, is a good soil for 

 corn and apples, but also produces the most excellent cheese in Eng- 

 land, as Mr. Woodward has pointed out. Along the Severn bank it 

 furnishes the double Gloucester ; at Melton Mowbray and Leicester it 

 produces Stilton; and in Somersetshire it unites with the triassic red 

 marl to yield the Cheddar. The fruitful vales of Eversham and Glou- 

 cester belong to this formation. The Oolite gives us the rock known 

 as cornbrash, which disintegrates into a splendid wheat-bearing soil, 

 naturally manured by its large quantities of phosphate of lime, the 

 so-called bone-earth. The Oxford Clay, on the other hand, is poor 

 and hard to cultivate, so that most of it lies under permanent pasture. 

 It forms the sheep-feeding vale of Blackmore, in Dorset. The Kim- 

 meridge Clay, in like manner, does not repay cultivation, and is mostly 

 employed for meadow or woodland. The Wealden, forming the great 

 trough between the North and South Downs, is another of the infer- 

 tile soils. It remained a great wood, the Andredesweald, or Forest of 

 Anderida, for a long period after the English conquest, and the local 

 names of the district still retain their forestine terminations of hurst, 

 ley, den, or field. Even at the present day the Weald is damp and 

 clayey land, little tilled, and either laid down in pasture or given over 

 to furze and heath. The Gault makes good grazing-lands, and the 

 Upper Greensand is in every respect a fertile formation. These two 

 series yield the rich Vale of White Horse, through which the Great 

 Western Railway runs between Swindon and Didcot, as well as the 

 vale of Aylesbury, whose name has become synonymous with pure 

 milk. The Chalk supplies us with South Down mutton, said to owe 

 its excellence not so much to the pasture itself as to a small land-snail 

 {Helix virgata) which the sheep devour in great numbers.* The Lon- 

 don clay, though stiff, can be made to yield good crops. Drift forms 



* These little mollusks themselves abound upon chalky soils, and are found nowhere 

 else, because they require large quantities of calcareous matter to form their banded 

 shells, while other species with more horny coverings live on soils where less lime can be 

 obtained. No snails can inhabit the limeless district of the Lizard in Cornwall. So 

 minute are the interpendcnces between every portion of organic and inorganic nature. 



