180 Causes of Fertility or Barrenness of Soils. 
even marly towards the lower parts, contain no flints, and form 
very fertile soils. Various theories have been started to explain 
this difference : doubtless to a considerable extent it may be 
traced to the proportion of clay present, but also to the existence 
of alkalies and silica in a comminuted form, either combined as 
soluble silicates or existing separately in such a state as to become 
slowly soluble under the action of rain water. It has been cal- 
culated fi"om analysis that about the same per-centage of silica is 
diffused through the lower chalk as is found collected together 
as flints in the upper.* To whatever due, the fertility of the 
lower chalk is so marked that in many Instances it has been even 
brought from long distances at considerable expense and applied 
as a dressing to the soils of the upper beds. The soils upon the 
oolite formation form the other principal example of calcareous 
origin : they occur in narrow patches, from Somersetshire through 
the central part of England, up to Yorkshire, generally forming 
elevated land. Resting on a porous subsoil, tbey seldom require 
drainage. The depth varies according to the situation, whether 
on the hills or in the valleys, and the particular bed from whence 
derived ; generally speaking shallow, but often resting on a 
brasiiy subsoil, capable of improvement by subsoiling. In the 
valleys occur useful soils, producing good cereal and leguminous 
crops ; but on the hills the soil is thin, porous, and poor, re- 
quiring different management. Seed crops must be occasional 
only, and the land is usually rested with sainfoin, which remains 
down several years, and being a leguminous fodder-crop, seems 
specially adapted for calcareous soils. Owing, however, to the 
variety of beds which occur in close proximity, and often come 
to the surface on the same farm, the nature of the land varies on 
the most limited areas, — a circumstance of great value to the 
cultivator, who can adapt his cropping to the peculiarities of each 
* The difference in composition of the two beds is well seen by the following 
tables: the 1st is an analysis by Playfair, of the upper chalk from Norfolk; the 
2nd, by Way, of the lower chalk from Farnham. 
No. 1. 
No, 
, 2. 
0-70 
95-50 
66- 
44 
Silica (partly as Silicate of Potasli) . . 
0-51 
26- 
09 
1 • 70 ) 
•64j 
3- 
04 
•37 
•10 
3- 
75 
•19 
08 
•13 
100-00 
100-00 
