Farming of Cornwall. 
421 
Sithney, and those in the western parts of Mylor, Kenwyn, and 
Kea, are of the same character as those already described, as 
resting on coarse argillaceous slates abounding in porphyritic and 
felspathic rocks (72). Those lying on the slates in the parishes 
of St. Withiel, St. Wenn, and Lanivet, are of a similar character, 
and likewise the soils in the eastern parts of Ladock, and western 
parts of St. Stephen's (68), and the slate soils of Warleggan, 
St. Neot, and St. Cleer (68). Most of those soils are siliceous, 
existing in the soils and clays in the form of quartz fragments, 
which intersect the slate rocks. The whole of these soils have 
been classed by Dr. Boase, in his contributions towards a know- 
ledge of the geology of Cornwall, as belonging to the porphyritic 
series, in contradistinction to those abounding in trappean rocks 
of greenstone, &c., which he has named calcareous. But this 
distinction is by no means clearly defined, and is neither geologi- 
cally nor chemically correct. 
Drainage, Sfc. 
33. We have in Cornwall two great systems of valleys, accord- 
ing to Dr. Boase's survey, " one running parallel with the central 
granite range, and the other intersecting the longitudinal valleys 
at right angles ;" and as all the intermediate hills are m.ore or 
less rounded, the country exhibits an undulating surface, which 
has already been compared to the waves of the ocean. Through 
these two great systems of valleys the fresh-water rivers flow, 
seeking outlets into the sea by the nearest continuous descent ; 
receiving tributary streams from the lateral valleys which they 
intersect. On the granite districts the drainage is occasionally 
obstructed, so as to produce accumulations of clayey matters from 
the washing of the hills; and sedgy pools, and occasionally mo- 
rasses or bogs are formed : these may be regarded as '•' alluvial 
deposits " of the modern kind. Very little in the way of drainage 
has been done towards reclaiming this kind of land ; the prin- 
cipal operations being confined to the slate formation, which, 
from the peculiar formation of the hills, the eminences sometimes 
closely approximating, furnishing narrow passages for the rivulets, 
as well as from the porous nature of the rocks, affords no sites for 
the accumulation of water by broad expanse of hollow land. 
Here and there, where the breadth between the hills may be from 
100 to 150 yards or more, marshy or moor land is found. 
There is seldom much difficulty experienced in obtaining a 
proper inclination for the discharge of water by drains in these 
cases ; and when properly executed, an exceedingly rich and vege- 
table mould is formed, making the richest and most valuable 
meadow-land in the county. The drains in these cases vary from 
3 to 6 feet in depth, and from 18 inches to 2 feet wide at the top, 
according to the depth, and from 16 to 18 inches at the bottom. 
