MINERAL MATTERS SUPPLIED BY IRRIGATION. 375 



Eose, not so much of moistening the ground, as of conveying to the 

 md in the valleys the mineral detritus washed from the moun- 

 tains at the time of the melting of the snow. This method of 

 artificial manuring by irrigation is commonly applied also in 

 those countries where there is no lack of rain and dew. It sub- 

 serves the same purpose as the mud of the Nile in Egypt, viz. to 

 replace the action of farm-yard manure. Where the mineral con- 

 stituents removed by a long succession of crops are not restored to 

 the ground either by animal manure, or by irrigation, the soil is 

 almost completely drained of its productive powers, as is the case, 

 for instance, in certain parts of the extensive tablelands of Tacun- 

 ga and Ambato (in the South American State Ecuador), where 

 barley will often barely give a two or threefold return, notwith- 

 standing the frequent alternations of rain and sunshine. From the 

 most reliable information obtained by me, even the most fertile 

 estates in San Salvador and Ohiriqui, in 'Central America, with 

 their most fruitful, loose, trachytic soil, abounding in potash and 

 silica, cannot show a single field on which maize has been grown 

 for thirty years running without a considerable reduction of prod- 

 uce a fact which sufficiently refutes the old mistaken notion of 

 the inexhaustible fertility of the soil in the tropics. 



On the western coast of Peru only those parts are extremely 

 sterile, where no little artificial canals convey to the dry soil the 

 water from the torrents of the Andes, which carries with it the 

 mineral detritus washed from the declivities of the mountains. 

 Wherever such artificial canals exist, and the conditions of the 

 ground are favourable, the soil on the coast as well as in the interi- 

 or of Peru and Bolivia is almost as productive as in the interior of 

 the highlands of Ecuador, New Granada, and Gautemala. But it 

 is not the water which is the agent in maintaining the steady pro- 

 ductiveness of the soil, but, as in the case of the Delta of the Nile 

 in Egypt, it is the mud carried along with the water, and which 

 has been washed away from the disintegrated rocks of the Andes. 

 The constituents of this mineral detritus, which are partly con- 

 tained in the water in a state of minute mechanical division, and 

 partly held in chemical solution, are brought to the fields by small 

 channels. The water thus conveyed from the mountains in innu- 

 merable furrows is soon absorbed by the soil or evaporated, leav- 

 ing a rich fertilising deposit behind. Pure rain water would be 

 of very little avail, as, for instance, in the extensive tableland of 

 Tacungar, with its barren pumice stone fields, where quite near 

 the equator rain pours down almost daily during nine months of 

 the year. It is not the atmospheric water that acts as the fertil- 

 ising agent, but the muddy streamlets from the Andes. In Peru 

 the fertilising action of guano is more enduring than in England, 

 because the potash which the guano does not restore to the soil, is 

 there supplied in the detritus from the trachytic constituents of the 

 Andes ridge, which abound in felspar. This natural mineral ma- 



