Cxlvi PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
which consists of pulverised and saturated crystals of various sili¬ 
cates, but particularly silicate of alumina. As the first of these has 
no cohesion whatever, and as the latter has an excess of cohesion 
but no porosity, it is obvious that, apart from chemical considerations 
altogether, a soil, to possess the necessary mechanical properties, 
should contain some of each of these two classes of ingredients, and 
that they should be intimately mixed together. 
This is precisely what Nature is constantly doing. The quartzites, 
grits, and sandstones are being broken down into sand; the granite 
is being crumbled into fragments, its quartz going to form sand and 
its felspar and mica to form clay; the mica-schists, the syenites, the 
quartz-porphyries, etc., in like manner go to form both classes of 
soil; while the basalts, porphyrites, felsites, clay-slates, tuffs, etc., go 
to form more or less impure clays. At the same time the mixing 
process is going on also, for the rain and the water-courses are 
passing over each of these rocks in turn and carrying off the loosened 
particles, to re-distribute them on the lower ground, not in hetero¬ 
geneous confusion, but to a large extent in definite sequence, accord¬ 
ing to their respective specific gravities. Thus it comes about that 
each locality receives its own distinctive kind of soil, some being 
more particularly suited for one kind of crop and some for another 
kind, but nearly all being susceptible of cultivation with the aid of 
slight modification at the hand of man. 
When we speak of the modification of soils through human 
intervention, we must bear in mind that the soil in its natural state 
is quite capable of supporting and nourishing plants in their natural 
state. It is only when we force the grass seed to the abnormal size 
of the grain of corn, in order to meet the requirements produced by 
over-population, that we have to supplement the natural nourishment 
contained in the soil. As a matter of fact, however, plants, even 
those under cultivation, and still more those in their natural state, 
derive the greater part of their nourishment from the atmosphere and 
from the water which they take into their tissues. The function of 
the soil therefore is mechanical rather than chemical, and its com¬ 
parative excellence consists more in the way in which its particles are 
arranged than what they are composed of. These particles must 
be in such a state of sub-division as to allow the rootlets of plants 
free access in all directions, and yet they must not be so far apart as 
to destroy the capillarity of the soil, that is, its power of absorbing and 
retaining moisture. 
Applying now the principles which I have indicated to the soils 
of Perthshire, we may express the characteristics of each, broadly, as 
follows. The soils of the Highland district, being derived chiefly 
from the metamorphic rocks, are arenaceous, or sandy, rather than 
argillaceous, or clayey. The clay which they do contain is not so 
rich in the nutritious earthy salts, such as soda, potash, and magnesia, 
as the clays derived from volcanic rocks. Their chief deficiency, 
however, is more frequently want of depth, so that in a dry season 
they have not sufficient underground source of moisture to draw 
upon. They are generally fairly warm and open. In the soils 
of Strathmore, the arenaceous element still predominates, but there 
