132 PHYSIOGRAPHY. fCHAP. 



the land, must have been much greater than that which is 

 now witnessed. 



The detrital matter which is worn away from the land, 

 and carried along by rivers, contains materials of every 

 degree of coarseness. It often happens that fragments of 

 rock, perhaps of considerable size, are loosened from cHffs 

 near a river by the action of rain and frost, and tumble 

 down into the stream. There they get slowly worn down, 

 by constantly knocking against each other, and may ulti- 

 mately be rubbed into the form of smooth round pebbles. 

 In the basin of the Thames, it is common for the hard 

 flints from the chalk to get broken and rolled about in the 

 water, and it is in this way that gravel is formed. The 

 substance which is strewn over our roads and garden-walks 

 consists, chiefly, of little bits of flint, which have been so 

 rolled about in water that the sharp points of the broken 

 stones are rounded off. All gravel has not however been 

 subject to the same amount of rough usage, so that whilst 

 the pebbles are in some cases well rounded, in other 

 cases they retain more or less of their angularity, though 

 the corners are never quite sharp. The small pieces worn 

 off the fragments of rock, as tliey rattle together on the bed 

 of the stream, get rolled about until they form small 

 rounded grains known as sa}id. As a rule, both the gravel and 

 the sand consist, chiefly, of the substance called silica, or the 

 material of which flint is formed, and which is chemically the 

 same as the matter of pure rock-crystal (p. 58). The gravel 

 and the coarser sediment are pushed along the bottom of 

 the river by the motion of the stream, whilst the finer sand 

 may be carried in suspension, though it will not travel so far 

 as the lighter particles of mud. The heavier pieces naturally 

 fall to the bottom first, so that if a quantity of mixed gravel, 

 sand and mud be shaken up in water, it will be found that 



