



56 THE WONDERS OF GEOLOGY. Lect. I. 



that rivers are the great natural outlets by which the super- 

 fluous moisture of the land is conveyed into the grand 

 reservoir, the ocean. And so exactly is the balance of 

 expenditure and supply maintained, that all the rivers on 

 the face of the earth, though constantly pouring their mighty 

 floods into the ocean, do not affect its level in the slightest 

 perceptible degree; we may therefore assume that the 

 quantity of moisture evaporated from the surface of the sea 

 and descending on the earth in rain and vapour, is exactly 

 equal to the sum of all the water, in all the lakes and rivers 

 in the world.* But though the quantity of fluid poured by 

 the rivers into the basin of the ocean is again removed by 

 evaporation, yet there is an operation silently and constantly 

 going on, which becomes an agent of perpetual change. 

 The rivulets which issue from the mountains are more or 

 less charged with earthy particles, worn from the rocks 

 and strata over which they flow; the united streams in 

 their progress towards the rivers become more and more 



* The quantity of water that percolates into the earth from the 

 surface is very great. It is well known that the water in mines varies 

 with, and depends upon, the rain falling in the districts where they 

 are situated. Seasons of heavy rain are followed by a great increase 

 of water in a mine, and the reverse happens from a drought. The 

 time which elapses before the effect of these causes manifests itself 

 varies considerably, and depends on the mineralogical character and 

 physical features of the country. In carboniferous limestone districts 

 the percolation of rain-water is particularly rapid, so that a heavy fall 

 of rain will often overpower the machinery sufficient for ordinary 

 drainage. " In two mines in Flintshire, the steam-engines, after having 

 been increased in speed from three or four strokes per minute to the 

 extent of their power, about five times as many, were yet incapable of 

 preventing the water from rapidly increasing and filling the workings. 

 In these mines there are great cross courses or faults, which can be 

 traced several miles, and pour torrents of water into the veins ; they 

 are, in fact, channels for subterranean rivers, and the latter carry with 

 them large quantities of sand and gravel, worn away from the rocks 

 through which they pass." — Mr. Taylor. 



