PROCEEDINGS-PERTHSHIRE SOCIETY OF NATURAL SCIENCE. Cxlv 
It tells us that as, in the organic world, all presently existing forms of 
life have been evolved from earlier and simpler forms, so, in the 
inorganic world, all the present features of the earth’s surface, 
whether solid or detached, have resulted from the gradual decay and 
reconstruction of matter, under the action of certain definite laws. 
In other words, all soils, as well as the solid rocks themselves, are 
the result of a gradual and orderly process of growth. If we take a 
district, such as our own county, and examine the vertical distribution 
of its soil-cap, we have at once a clue to this process. On examining 
sections at different altitudes, we find that the tops of the mountains 
are practically devoid of soil, except in crannies, where patches of 
lichen have loosened some particles from the rock, or where patches 
of alpine plants have sifted some of the fine rock-dust from the wind, 
and imprisoned it under close cushions of herbage. Further down 
the hillside a thin covering of soil begins to appear, kept in its place 
by the roots of heather and grass. In deep and abrupt gullies a 
comparatively thick deposit is frequently found, due to the fact that 
such gullies formed traps to catch and retain the debris from the 
travelling ice in glacial times. Coming now into the more open 
mountain valleys, we find that the floors of these consist of beds of 
soil of considerable thickness, but still very light and sandy in their 
nature. Descending next to the lowland part of the county, we find 
the wide plain of Strathmore occupied by deposits of sandy loam, 
generally of a dark red colour, and fairly uniform in composition. 
Lastly, coming to the lower reaches of the river, we have the stiff 
clays of the Carse of Gowrie. Now, if we take samples of each of 
these soils, and examine them carefully in the laboratory, we shall 
find that they have all certain features in common, but that each 
presents certain characteristics which indicate its origin. 
In the first place, we shall probably be surprised to find, on 
chemical analysis, that even the richest of them contains but a small 
proportion of organic matter—that is, matter derived from the decay 
of animal and vegetable substances. The proportion will probably 
not exceed 6 or 8 per cent., or, in very exceptional circumstances, 
io per cent. In the poorer soils from the upper districts it may 
not exceed 2 or 3 per cent. We see, then, that in every hundred 
parts of soil from 92 to 98 parts consist of inorganic or mineral sub¬ 
stances, and these have been derived wholly from the decay and 
trituration of the rocks which form the solid framework beneath. If 
we next examine the inorganic residue, after having removed the 
water and organic matter by drying and burning in the open air, we 
shall find that the mineral constituents of which it is made up differ 
in the different samples, both in regard to their chemical composition 
and their mechanical form and arrangement. By far the largest 
ingredient, of course, is silica, both free, in the form of sand, and 
combined with the various bases to form the silicates of alumina, 
soda, potash, etc. The fundamental difference between one soil and 
another lies in the varying proportions in which these two forms of 
this mineral are present. At the one extreme we have the white 
sand of the sea-shore, which consists entirely of minute particles of 
pure silica, while at the other extreme we have china or brick clay, 
