40-1 
Farmwf) of Cornwall. 
which by decomposition gives origin to a peculiar soil, distinct in 
its nature and character, and requiring in some instances an ap- 
propriate system of cultivation — these are — 
(1.) Granite, giving origin to the " groican soil.'' * 
(2.) Serpentine, belonging almost exclusively to the Lizard dis- 
trict. 
(3.) Schist or clay slate, forming the largest proportion of land 
in the county. 
(4 ) Carbonaceous deposits. 
Besides which, dykes of porphyry, beds of greenstone, hornblende, 
and other trappean rocks, and veins of quartz and limestone will 
claim a portion of our attention as they effect an influence upon 
the soils beneath which they occur. 
Granite. 
7 . The fertility of the soil on this formation depends greatly on 
the degree of its elevation and exposure. This is beautifully illus- 
trated by comparing the Roughtor and Brownwilly district with 
that of the Land's-End, the former being 1296 and 1368 feet above 
the level of the sea — a greater part of which exhibits scarcely any- 
thing else but sterility and desolation, on which fogs are frequent 
and snow often falls before it is seen in the lower grounds, whilst 
the height of the latter district rarely exceeds 300 feet, and snow 
is seldom seen. Dr. Paris has observed that the relative fertility 
of this soil depends also upon its mineral structure. Granite is 
composed essentially of three minerals — quartz, mica, audi felspar 
— its fertility chiefly depends, and is in proportion to the predo- 
minancy of felspar, due allowance being made for the varied 
conditions of weather to which it is exposed. Sometimes the 
quartz predominates, so as to constitute two-thirds or three-fourths 
of the whole rock — which difference, although overlooked by the 
geologist, produces important differences in the agricultural cha- 
racter of the soils, derived from their crumbling substances — the 
disintegration of which depends chiefly on the deconipositioi\ of 
the felspar. The felspar consists chiefly of a silicate of alumina 
in combination with a silicate of potash — the alkali forming the 
chief fertilizing ingredient ; and hence soils of this character arc 
celebrated for their abundant crops of grass. When this sub- 
stance is exposed to the action of the water charged with carbonic 
acid, it slowly suffers decomposition, in such a manner that on 
the sides and slopes of hills the greater part of its alkali is re- 
moved altogether, and hence the hill sides on this formation are 
invariably barren, the clay being washed away, and nothing left 
* Growan is the Cornish name lor gravel. 
