Cxlii PROCEEDINGS-PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
farmer. At the very base, however, of the sciences which are of 
importance to the agriculturist is geology, for without at least some 
knowledge of the geological structure of his district he cannot have 
a complete acquaintance with the nature and requirements of the 
various soils on his land. 
Before discussing the origin of soils and their distribution, it will 
perhaps enable you to understand the subject if I describe, from 
actual observation, four typical examples, in different parts of the 
county. 
i. The first I shall take is a section opened by the side of the 
road between Pitlochry and Moulin. Here a cutting is exposed in 
a bed of ancient river material,—sand, sandy clay, and gravel,— 
probably laid down after the close of the Glacial period. This bed is 
some twelve or fifteen feet thick, and scattered through it are pebbles 
and boulders of various sizes up to six or eight inches in diameter, 
and of various rock material, such as mica-schist, quartzite, hornblende 
rock, granite, diorite, etc. The bed has been cut down vertically 
with shovels to make top-dressing for the roads, and, strange to say, 
these boulders, consisting of some of the hardest crystalline rocks 
known, have been cut through also, just as if they had been raisins 
in a pudding. When we examine these boulders more closely we 
find that they are as soft as the clay itself, and that we can put our 
knife into each of them quite easily. 
Now, leaving this section for a few minutes, let us go up the 
road, past Moulin, and on to the moorland road which crosses the 
shoulder of Ben Vrackie. Here, by the side of the road, we come 
upon similar boulders of various crystalline rocks, but so hard that 
it is with the utmost difficulty that we break off the smallest chip 
with our geological hammer. What is the cause of this marked 
difference in rocks which were originally identical ? In the former 
case the cohesion of the constituent particles has been entirely 
destroyed, partly by the rending action of freezing and expanding 
moisture and partly by the solvent action of percolating water, 
charged with carbonic acid; but, at the same time, the boulder has 
been forced to retain its original form owing to the surrounding sand 
and clay acting as a retaining mould. The boulder on the hillside, 
on the other hand, has been subjected to the same breaking-up 
forces, but each particle as it was loosened has been carried off in 
turn either by the rain or the wind, so that what now remains is 
only the core of what was formally a much larger boulder. In other 
words, the boulder in the bank illustrates disintegration alone; the 
boulder on the hill illustrates disintegration plus denudation. In 
these we have types of the two great classes of soil of which I shall 
have to speak, namely, those which have been formed by accumu¬ 
lation in situ and those formed of material transported from a 
distance. One other feature about this section we note before 
leaving it, namely, that on the top of the sand and gravel rests 
a cap of dark brown vegetable mould about ten inches in thickness, 
in which grass, weeds, and broom are growing. The roots of these 
plants penetrate in many places through this vegetable mould into 
the sub-soil below for a distance of a foot or more. Further, we 
