AQUEOUS AGENCIES. 41 



along its burden of sand and gravel and shingle, every par- 

 ticle "becomes a tool which grinds, and is in turn ground 

 down in the double process of attrition and erosion. Every 

 runnel and rivulet wears for itself a channel, and bears the 

 eroded material down to the river ; the river performs the 

 same operation, but on a larger scale, and with marked in- 

 tensity during floods and freshets, cutting out ravines and 

 gorges, or scooping out broader valleys, and transporting the 

 debris to the lower levels of lakes, estuaries, and the ocean. 

 There the mud and sand and gravel borne from the higher 

 grounds come at last to rest, subside as sediments, and are 

 thus spread out as alternating strata, to be consolidated by 

 pressure, chemical agency, and other means, and ready, when 

 the event happens, to be upraised as the rock-formations of 

 newer lands. In like manner, also, with the waves and 

 tides and currents of the ocean. Restlessly and for ever 

 eating into and undermining the sea-cliff, the waves encroach 

 upon the land, pound down the hardest material to shingle 

 and gravel and sand, and this with rapidity according to 

 the nature of the opposing cliff, and the manner it is dis- 

 posed to the impact of the breakers. The effects of wave- 

 action are perceptible along every exposed shore ; here in 

 the undermined and falling cliffs, there in caverns and 

 gorges, and in another part in the "needles" and outstand- 

 ing rock-masses that have been severed from the land. 

 What the waves have worn down the tidal ebb and flow 

 disintegrate still more, and scour and carry away to the 

 stiller depths and more sheltered recesses. And the great 

 ocean-currents, too like the Arctic with its burden of ice- 

 bergs and rock- debris, or the Gulf Stream with its drifted 

 sea-weeds and animal exuviae are also incessantly trans- 

 porting and reasserting. Everything, however, comes at 

 last to rest in the waters, being either piled on shore as 

 sand, gravel, and shingle, deposited as silt in the deeper and 



