THE VOICES OF RIVERS. 29 



It is almost superfluous to point out ho\v necessary this con- 

 stancy in the meteorological character of a country is to the 

 well-being, or even to the existence, of its indigenous plants and 

 animals. If after prolonged periods of continual showers, such 

 as take place in the West Indies, equally long periods of an 

 African drought were to follow in our country, how would then 

 our fields and meadows be able to sustain so many millions of 

 men and of domestic animals ? 



Our whole agricultural system, the whole organic life of our 

 island, is based on the alternations of moist and dry weather 

 which distinguish our moderate climate. 



If the earth were everywhere covered with impermeable strata 

 of rock, the rains would either flow off so rapidly, or be so long 

 retained in extensive swamps and pools upon the surface, as 

 greatly to diminish the variety and luxuriance of vegetation. 



But by far the greater part of the earth-rind is composed of 

 alternating beds or strata of impermeable clay and porous lime or 

 sandstone, originally deposited in horizontal layers at the bottom 

 of the primeval seas, but now more or less displaced and set on 

 edge by the volcanic forces, which have so frequently changed 

 the surface of the earth. 



Wherever permeable beds crop out on the surface, the residuary 

 portions of rain-water, which are not disposed of by floods or by 

 evaporation, are absorbed into the fissures and small interstices 

 of the porous soil, and descending into their lower depths until 

 they reach an impermeable stratum, form the subterraneous 

 sheets or reservoirs of water from which our springs and rivers 

 are chiefly fed. The granite, gneiss, porphyry, lava, and other 

 unstratified and crystalline rocks of igneous origin, which cover 

 about one-third part of the habitable globe, are likewise inter- 

 sected by innumerable fissures and interstices, which collect and 

 transmit rain-water, and give origin to springs. 



Thus the volcanic forces, which, in the course of countless 

 centuries have moulded the earth-rind into its present form, 

 have at the same time furnished it with the necessary filters, 

 drains, conduits, and cisterns, for the supply of the sources, 

 brooks, and rivers that run along its surface. 



The geological convulsions of the globe, the evaporation 

 of the ocean, the circulation of the waters over the surface of 

 the earth, thus all harmoniously tend to the maintenance of 



