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JOURNAL OF THE PLYMOUTH INSTITUTION. 
uniform in the shillat, which accounts for the frequency of very- 
moist areas, both during summer and winter, on the east and west 
flanks of rising grounds. The west face of North Hill is much 
more damp than the north or south faces. 
This suggests the difficult subject of percolation through soils. 
It is true values may be had for many typical soils and subsoils ; 
but to apply such to a whole district or watershed without the 
greatest caution is dangerous. Yery valuable information would 
be had by dividing rivers into similarly featured districts or 
lengths, and dealing with the differences of discharge for evapo- 
ration and percolation. Whether rain-water flows off in floods, 
or in more regular streams, depends to a large extent on the 
soil. Our experience of the common filter will tell us that, with 
a certain head, the rate of filtration depends on the depth of 
filtering medium, and on the fineness of the top layers. Allow a 
film of mud to form on the surface, and the filtration through 
the whole series, however porous, will be arrested. The herbage 
on the surface affects the meteorology of a country more than 
any other temporary and accidental feature. Meadow land has 
a tendency to increase percolation and evaporation greater in 
amount than forests relatively to its bulk. This is mostly due to 
its open cellular structure. Recent experience in irrigation has 
established beyond a doubt that the depth of top-mould or humus 
is increased and improved generally by irrigation, apart from the 
effects of sewage which the water may contain. Water, in de- 
scending through the soil, forces along with it the air in the 
ground, discharging it into the drains or other openings. On 
subsiding, a suction is set up, which can only be drawn on from 
the fresh atmospheric air. Thus the closest of soils are oxidised. 
Many owe their major portion of oxidation to this one process ; 
and to this also rain owes most of its fructifying power. When 
the materials separated by oxidation, as in dunstone, are those 
favourable to growth, a rich soil is the result. Slate rocks do not 
decompose readily into a soil, although much softer, and more 
easily worn down, than either limestone or dunstone. 
II. Limestone District. 
At the junction of the limestone and slate rocks there is no very 
abrupt break. For instance, in the Railway cutting of the Sutton 
Harbour Branch at Laira Road Bridge, which is just on the division 
