On the Agricultural Geology of England and Wales. 447 



geological maps ; and if not, then upon what conditions the varia- 

 tions depend. 



3. The proper depths and distances of drains depend on the 

 permeabilitj of the soil and subsoil by water. A knowledge of 

 this and of the phenomena of springs, which is essential to 

 economy and efficiency in freeing land from water when it is 

 redundant, and in obtaining a supply of it when deficient, con- 

 stitutes one department of agricultural geology. 



4. Another comprises a knowledge of the nature and pro- 

 perties and the distribution of mineral manures, or those sub- 

 stances which m.ay be obtained from the subsoil and substrata for 

 the correction of chemical and mechanical defects in the compo- 

 sition of the soil. It comprises also the establishment of a definite 

 nomenclature for these, instead of that at present in use, which is 

 as vague and unsatisfactory as the nomenclature of soils. When 

 we are told of the benefits which have resulted from the applica- 

 tion of marl or clay in a given district, who beyond the limits of 

 the district can form any idea of the composition or geological 

 relations of the substances to which the terms are applied, except 

 that they are not farmyard manure, but that they are dug out of 

 the earth, and that they are not caustic lime ? Instances might 

 be adduced in which the clay of one county, or one side of the 

 same county, is the identical substance which is called marl in 

 another. 



5. Elevation above the sea, jointed structure and slaty cleavage, 

 impregnation with metallic substances, and that metamorphic or 

 altered condition which arises from contact with granite and other 

 igneous masses, are local accidents to which the strata have been 

 subject, and which must affect the character of soils derived 

 wholly from rocks of the same formation. Soils, for instance, on 

 the carboniferous limestone, which is spread out in broad undula- 

 tions, at low levels, over so large a portion of Ireland, must have 

 a very different value from soils on the same rock, at elevations of 

 1500 and 2000 feet, to say nothing of greater heights, in York- 

 shire and Derbyshire. Soils also will be drier on rocks traversed 

 by numerous joints and by slaty cleavage than on those which are 

 destitute of them. Thin soils resting on such rocks, if the joints 

 are numerous and the cleavage vertical or highly inclined, are 

 great devourers of manure, which runs through them like a sieve. 

 Salts injurious to vegetation prevail in soils resting on rocks which 

 abound in metallic sulphurets. The mineral characters of the 

 Devonian or old red sandstone rocks vary greatly in Hereford- 

 shire, Devonshire, and Russia, as they have or have not been 

 subject to those local accidents which induce metamorphic struc- 

 ture. In Russia, where thev have not been affected bv contig-uous 

 masses of igneous rocks, or by lines of disturbance, but are spread 



