178 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
21. Soil: its Origin and Nature. 
By Professor JAMES GEIKIE, LL.D., D.C.L., F.R.S. 
1 
It is matter of common knowledge that the so-called “ solid 
rocks ”—the bones or skeleton of the land—are to a great extent 
concealed under a covering of loose materials of various kinds. 
More especially is this the case in low-lying and gently-undulating 
countries, where hard rock may perhaps be seen only in river- 
cuttings, sea-cliffs or other natural exposures, or in artificial 
excavations. In mountainous regions, on the other hand, the 
bones of old Mother Earth are more commonly laid bare. Even 
in such tracts, however, the steeper hill-slopes and the bottoms 
of the valleys are often more or less overspread with incoherent 
materials—the living bed-rock appearing perhaps only in the 
dominant peaks, crests, ridges, and buttresses of the higher 
elevations, and in ravines and gorges at lower levels. All the 
unconsolidated materials referred to are known collectively as 
“‘soils” and “subsoils,” and consist essentially of mineral 
ingredients, derived from the disintegration of the so-called 
“solid rocks.” Such being the case, it is obvious that the 
character of subsoils and soils must vary with the nature of the 
rock or rocks from which they have been derived. Disintegrated 
sandstone, for example, must differ greatly from disintegrated 
granite. It does not follow, however, that the soil-cap yielded 
by any particular kind of rock, must necessarily always have 
the same character. Much depends on the configuration or 
shape of the ground, and much also on climatic conditions. 
This will be readily understood when we consider how subsoils 
and soils come to be formed. 
Rocks are invariably traversed by cracks and other fissures or 
natural division-planes, and they are moreover all more or less 
porous. Certain grits and sandstones, for example, are so 
pervious that water soaks into them almost as readily as into a 
sponge. Other rocks are so fine-grained and’ compact that they 
are only permeated with the greatest difficulty. So slowly, 
indeed, does water invade the extremely minute pores and 
capillaries of clays and argillaceous rocks, that these rocks are 
usually described as “impermeable.” 
The mechanical action of rain and frost upon rocks is so very 
obvious that it hardly needs to be described. Everyone knows 
