Or the Agricultural Geology of England and Wales. 449 



embrace two classes of facts — the distribution and composition of 

 the rock formations, and the distribution and composition of the 

 drifts or erratic tertiaries. Geolog'ists have occupied themselves 

 too exclusively with the former ; the latter, from their superficial 

 position and the extensive areas which they cover, are of the most 

 immediate importance in an agricultural point oi" view. There 

 are but few large tracts in Britain which are wholly exempt from 

 iheir influence. When they are only two feet thick they consti- 

 tute in many cases both soil and subsoil. When their depth 

 extends to seven feet, it is greater than that of the deepest drains 

 of the deepest drainers. There are many places in which these 

 deposits are several hundreds of feet thick^ and then the substrata 

 can have no agricultural value, except from the fossil manures 

 which are furnished by their exposure within accessible distances. 

 Even the fossil manures of the erratic deposits themselves are 

 rarely raised from beneath a head or overburthen of more than 

 twenty feet ; twenty yards is the maximum we ever knew. 



In our geological maps all these deposits are assumed as 

 removed, and that rock is exhibited as constituting the surface^, 

 which would in that case be the surface. Nor is this the only 

 defect of geological maps, as at present constructed, which de- 

 tracts from their value for agricultural purposes. Agriculture 

 requires a chemical, or, which is the same thing, a mineral classi- 

 fication of soils, Vv'hile the classification of strata adopted in these 

 maps is zoological. Their colours exhibit the areas occupied by 

 the outcrop of certain groups of strata, made up of many sub- 

 ordinate beds, differing in mineral composition, but united by the 

 bond of an assemblage of organic remains which is common to 

 the whole group, and distinct from other fossil groups higher or 

 lower in the series. For most purposes of geology such a classi- 

 fication is the best. A classification dependent on mineral cha- 

 racters had previously been tried, and found defective. We can 

 ascertain by its organic contents to what part of the series a given 

 rock belongs — by its mineral characters we cannot. The strati- 

 fied rocks, whose collective thickness amounts to several miles, 

 constitute a vast succession of strata, in which argillaceous, cal- 

 careous, and siliceous beds are repeated again and again. It is a 

 general but not invariable truth, that the argillaceous, calcareous, 

 and siliceous strata associated with the group of fossils which 

 characterise one part of the series, differ somewhat from those 

 associated with the peculiar organic remains of another. But 

 while there are these differences vertically in different parts of 

 the series, there are greater differences horizontally in different 

 parts of the same formation. Its mineral characters are fre- 

 quently changed in the distance of less than one hundred miles. 

 Its zoological characters are the same at points thousands of 



VOL. XII. 2 G 



