HO CAffONS. 



Some geological formations themselves form valleys or low ground, 

 traversed throughout their extent by rivers. This is especially the case 

 with the Triassic rocks. In North Wales, the Clwyd drains a valley 

 of New Bed Sandstone contained in a fold of older deposits. All the 

 latter part of the course of the Dee and the Mersey is through the New 

 Red Sandstone. A considerable part of the Severn traverses the same 

 formation. The Trent almost wholly drains this formation, though its 

 tributaries, the Dove and the Derwent, come down from escarpments 

 in the Carboniferous rocks of Derbyshire. The Ouse flows through 

 the Triassic rocks of the plain of York. In most of these cases the 

 valley drained by the river is so little depressed that it can scarcely 

 be regarded as a valley in the ordinary sense of the term. 



Valleys in Tablelands. River valleys, or valleys in plains, have 

 long been distinguished from the valleys which occur in tablelands or 

 elevated tracts. As regards their origin, they have nothing uncommon. 

 When portions of the earth's crust are compressed, and a part rises 

 upward, the adjacent part of the fold of necessity sinks downward, the 

 elevated part being a saddle, the depressed part a trough. As the 

 saddle rises in the sea, the waves cut its surface smooth, and as it rises 

 higher, it often becomes fractured through its thickness by lateral com- 

 pression. But at last, rising out of the sea as a plain, it begins to 

 experience the wearing or solvent action of the rain, which, descending 

 upon it, drains into the fissures, and makes them wider and deeper. 

 As elevation progresses, the land rises higher, and the narrow valley is 

 cut deeper and deeper. It happens that many tablelands are com- 

 posed either of limestones or volcanic rocks, and just as a mountain 

 brook, swallowed up by a fissure in the limestone, flows underground 

 and excavates a channel for itself, which eventually becomes a cavern 

 with steep sides, so the solvent power of the drainage-water, increased 

 by carbonic acid liberated from decaying vegetation, has enabled brooks 

 in mountain regions to cut deep and narrow gorges for themselves, 

 which characterise limestone scenery. For these valleys the Spanish 

 name canon has been adopted. Similar valleys, however, are formed 

 in some kinds of igneous rock. Thus, the broad sheets of lava which 

 extend through California and the Rocky Mountain region are traversed 

 by some of the most remarkable canons in the world. Similar valleys of 

 enormous depth extend through the tablelands of India. Though their 

 origin is the same as the valleys in the limestones, there is a slight 

 difference in the cause of their formation. The lava consists to a large 

 extent of felspars and minerals which contain small quantities of lime, 

 or soda, or potash. All these substances are capable of being taken 

 into invisible solution in water which is charged with carbonic acid ; 

 and when the rock parts with any of them, its nature is changed. 

 Previously it may have been hard enough to turn the edge of steel, 

 and not easily affected by the beating of rain, or the pounding of 

 stones which the torrent may have carried ; but no sooner does the 

 felspar part with one of its constituents than the rock becomes mud. 

 And then the water sweeps the impalpable particles away, deepening 

 the river channel, and emerges from the hills loaded with sediment. 



