30 THE VICTORIAN NATURALIST. 



itself, for here sand, gravel, and boulders are scrubbed against 

 the underlying rock and so rasp it away. Consequently, if no 

 other agency were at work, we should find all comparatively rapid 

 streams at the bottom of a precipitous-sided gorge, such as those 

 we read of as being occupied by the Colorado River in the 

 Western United States. But such precipitous valleys are rare, 

 and may be said to be confined to country in which the strata 

 are horizontal and consist of resistant rocks overlying others 

 more easily acted on by the weather. The constancy with which 

 we find the sides of valleys to have a gradual slope instead of a 

 precipitous one compels us to look for a constantly acting cause, 

 and this we find in the group of phenomena generally spoken of 

 as " weathering." The sides of the steep cliffs are broken down 

 and their material thrown into the river bed, to be still further 

 broken up and washed away. The causes which bring about this 

 result are, as has just been stated, manifold, and their causes in 

 turn are diverse. The rock, if it be a hard one, has first to be 

 disintegrated, so that it may be washed down by the ordinary 

 rainfall acting in purely a mechanical way. Rock is broken 

 down by several different agencies, and the question which will 

 be the more rapid in its effects will depend on the structure of 

 the rock and on the chemical composition of its constituents. 

 The action of percolating rain water with the acids produced by 

 the decomposition of organic matter, the expansion and contrac- 

 tion caused by alternations of heat and cold, the force exerted by 

 roots growing down cracks, all have a destructive effect, and 

 render the hillsides more liable to wash down by the rain. From 

 a consideration of these facts we see that, other things being 

 equal, the older a valley is the more gradual should the inclina- 

 tion of its sides be ; exposure of bare rock will be absent, and we 

 will find a grassy or even tree-clad slope. But before we apply 

 this criterion of age we must see that the other things are equal, 

 the nature of the rock and the amount of rainfall being the most 

 important. 



A clear grasp of the facts contained in these prefatory remarks 

 is necessary for the understanding of a particular case which came 

 recently under my notice. 



Visitors to Lome will remember that as a rule the various 

 creeks flow down valleys which are densely clothed by timber 

 and scrub ; the valley floor is covered with a tangled maze of 

 vegetation through which it is difficult to force one's way, and it 

 is mainly just below waterfalls that bare rock is much in evidence. 

 One valley, however, stands out in marked distinction from all 

 those that I have seen, and that is the valley of the Cumberland. 

 Here we find a narrow gorge, hemmed in between cliffs that in 

 places rise precipitously for hundreds of feet. The stream-bed is 

 swept clear of obstructing vegetation and the river for miles 



