§ 20. EFFECTS OF STREAMS AXD RIVERS. 55 



the chalk hills of Sussex, Surrey, Kent, and Hampshire, 

 were once the delta of a river, that flowed through a 

 country which is now swept from the face of the earth — a 

 country more marvellous than any that even romance or 

 poetry has ventured to portray. 



20. Aqueous agency : the effects of streams axd 

 rivers. — The operations we have now to consider are 

 produced by a substance, the most abundant in nature, and 

 with the properties of which we are so familiar, that we 

 but little appreciate the marvellous phenomena they present. 

 This substance — which in one state constitutes vast islands 

 and continents, forming masses that rival in transparency 

 and brightness the rock crystal or the diamond, and are more 

 durable than granite, and so sterile as to afford no suste- 

 nance, even to the simplest forms of vitality — in another 

 condition is invisible, and separates into two gases, which 

 supply heat and light to organic bodies ;— in a third state 

 it exists as an elastic vapour, which yields to man a power 

 far surpassing that of the fabled wand of the magician, 

 enabling him to cross the ocean in spite of the elements, 

 and traverse the land with a rapidity exceeding that of any 

 other animal ; — and lastly, it appears as a fluid which is the 

 essential support of animal and vegetable life, and covers a 

 large portion of the surface of our globe ; affording in its 

 profound abysses a habitation for the most colossal of ex- 

 isting animals, and containing in each drop myriads of 

 the minutest beings which the aided eye of man is able to 

 descry! — such are the wonderful properties of the substance 

 that, in its fluid state, we term water, and the geological 

 effects of which we now proceed to examine. 



In pursuance of this object, we will first notice the 

 changes produced on the surface of the land by the agency 

 of streams and rivers. I need not dwell on those meteoro- 

 logical causes by w r hich the descent of moisture on the 

 surface of the earth is regulated ; but will merely observe, 



