MANUAL OF AGRICULTURE. 235 



Oolite, lower — inferior oolite, fullers' eartli, great or Bath 

 oolite, forest marble; middle — Oxford clay, coralline 

 oolite; and tlie upper — Kimmeridge clay, Portland and 

 Purbeck beds. 



(c.) Cretaceous era — Hastings sand, Weald clay, lower green- 

 sand, gault, upper greensand, cbalk marls, and chalk. 

 III. Tertiary or Cainozoic loeriocl, including 



{a.) Eocene era — lower, middle, and upper, — the lower in- 

 cluding plastic and London clays, and the middle and 

 upper the deposits formed in estuaries. 



(&.) Miocene era — Lignites and leaf-beds. 



(c.) Pliocene era — Crag formations. 



(d) Pleistocene or post-tertiary era, comprising boulder clay 

 or glacial drift, raised sea beaches, fens, peat bogs, river 

 deltas, alluvium, sand dunes, and so on. 



These main groups, and, with a few exceptions, the various 

 strata they respectively comprise are all represented in the 

 British Islands, — an evidence of the extensive convulsions they 

 have undero'one. 



Although in few countries do they observe such an unbroken 

 series, still they invariably observe the cardinal order of deposi- 

 tion, whatever hiatus may occur in it. Periodic convulsion 

 throughout immense areas is evidenced by the existence of fossils 

 of land plants, which flourished on the soil of certain strata, 

 being discovered beneath immense stratification of a different 

 class. To the variety of the British rock formations are due the 

 multiplicity of its types of natural scenery, its many difterent 

 modes of agricultural practice — all included, too, within so small 

 a superficies. • This variation in practice is a consequence of the 

 variety of soil, which, as a rule, has a direct relation to its underlying 

 rock formation. By the term ''soil" is understood so much of the 

 surface as in cultivated ground comes under the operation of the 

 plough, and which in land still in a state of nature would come 

 under such infiuence were it to be cultivated: '' subsoil " is what 

 comes immediately under it. "Where identity of chemical con- 

 stituents does not exist between the soil and its subjacent rock 

 formation, the constituents of the former have been imported from 

 anotlier source. But in all cases, rock and su])soil alike have an 

 im])i)rtant bearing upon the questions of facility of drainage, the 

 physical features of exposure, flatness, or declivity of the soil. 



As already mentioned, the soils of the Laurentian, Cambrian, 

 and Silurian eras are mostly found at a high elevation ; and, in 

 addition, being for most part of a poor description, are seldom cul- 

 tivated with any degree of success. On the other hand, those of 

 the Devonian and its com]ianion series possess all degrees of 

 value. 



