164 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
tracts of land, although, where covered with grass or heather, a thin 
skin of turf or peat may be present. Generally speaking, however, 
the stratum in which the roots have to collect plant-food is a bed 
of more or less disintegrated rock, varying in depth and texture 
with the rock from which it has been formed. The harder the 
rock, the thinner we usually find this penetrable layer to be, while 
on soft rock, which easily breaks up by chemical and mechanical 
agencies, fresh portions of the solid rock are more easily detached, 
and become part of the subsoil. This difference in the character of 
rocks is of considerable importance in growing timber trees. The 
length of time which trees take to mature, and the persistent and 
far-reaching character of their root-systems, enable them to thrive 
where smaller and shorter-lived plants could scarcely subsist. 
While, therefore, the geological formation on which a soil rests 
may be practically without importance, it determines, in a great 
measure, the rate at which the transformation of rock into soil 
proceeds. As is well known, unprotected soils resting on sloping 
ground are often washed away by heavy rain at a faster rate than 
the weathering process of the rock beneath goes on. A thin soil, 
on easily weathered rock, is consequently of more value for timber- 
growing than the same depth of soil on a hard and imperishable 
foundation, because the presence of the trees not only prevents or 
retards denudation by rain, but also assists in the weathering process, 
through their roots penetrating cracks and fissures, and there acting 
both mechanically and chemically on the upper and loose strata. 
By far the greater part of the semi-waste land which calls for 
afforestation rests on metamorphic rocks composed of gneiss and 
mica-schist, both comparatively slow-weathering rocks. The latter 
of these, however, being of a more slaty texture, is frequently loose 
and crumbly, and a large proportion of this land is well adapted for 
the growth of coniferous timber, the valleys being frequently suit- 
able for agricultural purposes. While we do not consider it 
absolutely necessary that the geological character of the area 
should be minutely defined, it would nevertheless be an advantage 
to have it on land answering in every particular to the description 
roughly given above. As it is principally the Highlands of Scot- 
land which most need development, we should be affording a 
practical test of their suitability for afforestation by placing our 
forest upon the same class of soil as that formed from the rocks 
peculiar to the Highlands, although this, as all other matters, may 
be affected by circumstances, 
