242 CLASSIFICATION OF TIIF 3TRATIF1KD ROCKSj 



districts where each of these groups of rocks forms the surface, and the 

 general agricultural character of the soils that rest upon them. 



I. Tertiary Strata — characterized by containing, among other fos- 

 sils, the remains of animals, which are identical with existing species 



NAME AND THICKNESS. MINERALOGICAJL CIIARACTEKS. 



1°. Crag. 50 ft. A mass of rolled pebbles mixed with 



marine shells — resting on beds of sand 

 and sandy lime-stone ; the whole more 

 or less impregnated with oxide of iron. 

 Extent. — The Crag forms a stripe of land a few miles in width in the east- 

 em part of Norfolk and SuflFoUc, and in the soiith-eastem part of the latter coun- 

 ty. It is a flat, and generally, it is said, a fertile arable district. 



2°. Fresh-water Marls. 100 ft. Marls and marly lime-stones, with 



fresh-water shells divided into two se- 

 ries by an estuary deposit, containing 

 marine shells. 

 Extent. — On these beds reposes the soil of the northern half of the Isle of 



Wight, the only part of England in which they appear at the surface. 



3°. London Clay. 200 to 500 ft. Stiff, almost impervious, brown, blue, 



and blackish clay, rich in marine shells, 

 and containing layers of lime-stone no- 

 dules. 

 Extent. — The greater part of the county of Middlesex, the south-eastern 

 half of Essex, and the southern half of Hampshire, rest upon the London Clay. 

 Soil. — The soil is naturally strong, heavy, wet, and tenacious, "sticking to 

 the plough like pitch," and shrinking and cracking in dry weather. Where it 

 is mixed with sand, it forms a fertile loam ; and hence where the sand of the 

 subjacent plastic clay is easily accessible, it may readily be improved by ad- 

 mixture. Repeated dressings of London manure convert it into rich meadow 

 land, and even where this cannot be obtained, the difficulty and expense of cul- 

 ture have caused a very large portion of it to be retained in pasture. That 

 which is under culture is said to be too strong for turnips and barley, but to 

 grow excellent crops of wheat and beans. 



4°. Plastic Clay. 300 to iOO ft. Alternating beds of clay and sand, of 



various colours and thicknesses. Some 

 of the beds of clay are pure white, and 

 so fine as to be used for making pipes. 



Extent. — This formation surrounds the London clay with an indented, gen- 

 erally low, and flat belt, of varying breadtli, occupying a large space in Hamp- 

 shire and Dorset, in Essex, Suffolk, and Norfolk, — stretching along the north- 

 ern part of Kent and Surrey, and throwing out aiTns into Berks, Buckingham, 

 and Hertford. 



Soil. — The soil is very various, the alternate beds of sand and clay of differ- 

 ent qualities producing soils of the most unlike quality of\en within very short 

 distances. The greatest portion of this tract is in arable culture, but there are 

 extensive heaths and wastes in Berks, Hampshire, and Dorset. 



In Norfolk and Suffolk, where the lower beds of this sand rest upon chalk, 

 the soil is readily changed, by an admixture with this chalk, into a good sandy 

 loam, which will yield large crops of turnips, barley, and wheat, instead of the 

 heath and bent, its sole original produce. This chalking is gener£dly repeated 

 once in 8 years, at an expense of 50s. an acre. In Hampshire and Berkshire, 

 th,e same method is adopted with great success, and the rich crops now reaped 

 from Hounslow Heath are the result of this method cf imorovement. 



