440 
Farming of Cornwall. 
62. Magnesia, in combination with silica, forms nearly the 
entire mass of the serpentine rock (29). It is also found averag- 
ing 21 per cent, in hornblende, and 6 per cent, in the diallage; 
and in small proportions in the granite and clay-slate rocks ; 
united with phosphoric acid, it forms the most valuable mineral 
manure, — the ashes of all descriptions of grain containing it, par- 
ticularly the wheat plant, which cannot be grown without it. 
63. Lime is found largely in the hornblende rocks, which con- 
tain from 10 to 13 per cent. ; it is also contained in the dunstone, 
in nearlv all our trappean rocks, and in a great many of the asso- 
ciated slates. We have a very great proportion of lime in our sea 
sands, the feriilizing properties of which are valued according to 
their contents of comminuted shell ; the amount of carbonate of 
lime being a correct index of the quantity of shell, and of the 
feriilizing power in a given specimen. The following are the 
proportions of lime found in the sands from 14 different districts, 
by different chemists : — 
1. 
Gwithian and Phillack 
70 per 
cent, of carbonate of lime. 
2. 
Gannel (near the mouth) 
83^ 
5 » 
3. 
Forth Towan 
50 
5 > 
4. 
Portreath .... 
2.5* 
> > 
5. 
Perranzabuloe . 
70 
> 5 
6. 
St. Mawes 
64 
3 3 
7. 
Falmouth Harbour . 
80 1 
> 3 
8. 
Padstow sand . 
86ii 
3 3 
9. 
Harlyn Bay 
94 
3 3 
10. 
Trevose Bay 
911 
3 3 
11. 
Blown saud from Bude . 
68 
3 3 
12. 
Beach sand do. 
40 
3 > 
13. 
Stanbury Mouth 
52 
3 3 
14. 
Widemouth 
44 
3 3 
Besides carbonate of lime, these sands contain silica, alumina, and traces 
of oxide of iron. The shell also contains traces of" phosphoric acid. In 
Worgan's time the carriage of sea-sand was estimated at 32,000/. per 
annum. § 
* A large stream of water from the mines falls into the sea at Portreath, 
which has carried down in the course of ages the excess of worthless 
matters found in this sand. 
■I- This is chiefly all coralline deposit, found in the bottom of Falmouth 
Harbour. 
j; It is calculated from fair data that 100,000 tons are annually removed 
from Padstow Harbour only, and over the whole county no less than 
7,000,000 cubic feet are disturbed. 
§ In Worgan's time the expense for land carriage only wis estimated at 
32.000/. ; it is probably much greater now. Within the last two years a 
patent has been obtained for the purpose of calcining the north coast 
sand, this operation rendering it more soluble, and its action on the soil is 
thereby more quickly produced. 
