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 
