1 6 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



3. Soil : its Origin and Nature. 



By Professor James Geikie, LL.D., D.C.L., F.R.S. 

 {^Continued from Vol. XX. p. 182.) 



II. 



Soils are usually classified by agriculturists as {a) Sedentary 

 and (yh) Transported. These terms, however, are true only of 

 subsoils. All soils, properly so-called, may be said to be 

 sedentary, inasmuch as they are formed out of the immediately 

 underlying subsoils. To a geologist " soil " and " subsoil," no 

 matter how intimately related, are nevertheless distinct things. 

 Subsoil is an aggregate of disintegrated and decomposing rock- 

 material, while soil consists of similar materials, with the addition 

 of a varying amount of organic matter. True soil rarely exceeds 

 one foot in thickness, and is usually thinner ; but subsoil may 

 vary in depth from a few inches up to hundreds of feet. 



For this country a more suitable classification of soils would 

 be {a) Bed-rock Soils and {b) Drift Soils. Under the former 

 head would be included all soils overlying subsoils which are 

 in process of being derived from the breaking-up and dis- 

 integration of the bed-rock. The latter term, on the other hand, 

 would embrace all soils formed upon the surface of superficial 

 formations of every kind. These " superficial formations " 

 overlie and conceal the bed-rock, and are of very unequal 

 thickness and varied origin — the soils yielded by them diftering, 

 it may be, entirely from the subsoils and soils which the con- 

 cealed bed-rocks themselves would have supplied. A short 

 description of a few typical examples of bed-rock soils and drift 

 soils may suffice to illustrate the varying character of these two 

 great groups. 



{a) Bed-rock Soils. 



The bed-rocks of this country differ greatly as regards their 

 character and origin. Some consist of confused aggregates of 

 crystalline ingredients, and are obviously of igneous origin, 

 having cooled and consolidated from a molten condition. Such 

 are the well-known rocks granite and basalt. Others resemble 

 these in being crystalline, but differ from them in showing a 

 certain arrangement of their ingredients in more or less regular 

 lines or layers. Crystalline rocks having this leafy arrange- 



