CANE ASHES. CRUSHED AND DECAYED TRAPS AND LAVAS. 361 



burned, it can in many cases be applied with good effects as a top-dress- 

 ing to grass lands which are overgrown with moss ; while the admix- 

 ture of cinders in the ash of the less perfectly burned coal produces a 

 favourable physical change upon strong clay soils. 



h. Cane Ashes. — I may allude here to the advantage which in sugar- 

 growing countries may be obtained from the restoration of the cane ash 

 to the fields in which the canes have grown. After the canes have been 

 crushed in the mill they are usually employed as fuel in boiling down 

 the syrup ; and the ash, which is not unfrequently more or less melted, 

 is, I believe, almost uniformly neglected — at all events, is seldom ap- 

 plied again to the land. According to the principles I have so often 

 illustrated in the present Lectures, such procedure must sooner or later 

 exhaust the soil of those saline substances which are most essential to 

 the growth of the cane plant. If the ash were applied as a top-dressing 

 to the young canes, or put into the cane holes near the roots — having 

 been previously mixed with a quantity of wood-ash, and crushed il it 

 happen to have been melted — this exhaustion would necessarily take 

 place much more slowly. 



i. Crushed Granite. — We have already seen that the felspar existing 

 in granite contains much silicate of potash and alumina. It is, in fact, 

 a natural mixture, which in many instances may be beneficially applied, 

 especially to soils which abound in lime. It is many years since Fuchs 

 proposed to manufacture potash from felspar and mica by mixing them 

 with quicklime, calcining in a furnace, and then Wcishing with water. 

 By this means he said felspar might be made to yield one-fifth of its 

 weight of potash. (Journal of the Royal Institution, I., p. 184.) Mr. 

 Prideaux has lately proposed to mix up crushed granite and quicklime, 

 to slake them together, and to allow the mixture to stand in covered 

 heaps for some months, when it may be applied as a top-dressmg, and 

 will readily give out potash to the soil. Fragments of granite are easi- 

 ly crushed when they have been previously heated to redness, and there 

 can be little doubt, I think, that such a mixture as that recommended 

 by Mr. Prideaux would unite many of the good effects of wood ashes 

 and of lime. 



k. Crushed Trap. — I need not again remind you of the natural fer- 

 tility of decayed trap soils (Lee. XII., §4,) and of the improvement which 

 in many districts may be effected by applying them to the land. When 

 granite decays, the potash of the felspar is washed out by the rains, and 

 an unproductive soil remains — when trap decays, on the other hand, the 

 lime by which it is characterised is not soon dissolved out, so that the 

 soil which is produced is not only fertile in itself, but is capable of being 

 employed as a fertilizing mixture for other soils. Thus when it is much 

 decayed it is dug out from pits both in Cornwall and in Scotland, and 

 is applied like marl to the land. 



l. Crushed Lavas. — Of the fertile and fertilizing naturerf the crushed 

 or decayed lavas I ha^re also already spoken to you (Lee. XII., § 4). 

 In St. Michael's, one of the Azores, the natives pound the volcanic mat- 

 ter and spread it on the ground, where it speedily becomes a rich mould 

 capable of bearing luxuriant crops. . At the foot of Mount Etna, when- 

 ever a crevice appears in the old lavas, a branch or joint of an Opuntia 



