324 THE BEAUTIES OF NATURE CHAP. 



nean changes, but by far the greater number 

 are, in their present features, mainly the re- 

 sult of erosion. As soon as any tract of land 

 rose out of the sea, the rain which fell on the 

 surface would trickle downwards in a thou- 

 sand rills, forming pools here and there (see 

 Fig. 37), and gradually collecting into larger 

 and larger streams. Wherever the slope was 

 sufficient the water would begin cutting into 

 the soil and carrying it off to the sea. This 

 action would be the same in any case, but, 

 of course, would differ in rapidity according 

 to the hardness of the ground. On the 

 other hand, the character of the valley 

 would depend greatly on the character of 

 the strata, being narrow where they were 

 hard and tough ; broader, on the contrary, 

 where they were soft, so that they crumbled 

 readily into the stream, or where they were 

 easily split by the weather. Gradually the 

 stream would eat into its bed until it reached 

 a certain slope, the steepness of which would 

 depend on the volume of water. The erosive 

 action would then cease, but the weathering 

 of the sides and consequent widening would 



