Ch. VII.] ALLUVIUM. 83 



at b b, similar unrounded nodules of flint, still preserving their irregular 

 form and white coating, are found at various depths in the midst of the 

 loose materials filling the pipe. These have evidently been detached 

 from regular layers of flints occurring above. It is also to be remarked 

 that the course of the same sand-pipe, b 5, is traceable above the level 

 of the chalk for some distance upwards, through the incumbent gravel 

 and sand, by the obliteration of all signs of stratification. Occasionally, 

 also, as in the pipe d, the overlying beds of gravel bend downwards into 

 the mouth of the pipe, so as to become in part vertical, as would happen 

 if horizontal layers had sunk gradually in consequence of a failure of 

 support. All these phenomena may be accounted for by attributing the 

 enlargement and deepening of the sand-pipes to the chemical action of 

 water charged with carbonic acid, derived from the vegetable soil and 

 the decaying roots of trees. Such acid might corrode the chalk, and 

 deepen indefinitely any previously existing hollow, but could not dissolve 

 the flints. The water, after it had become saturated with carbonate of 

 lime, might freely percolate the surrounding porous walls of chalk, and 

 escape through them and from the bottom of the tube, so as to carry 

 away in the course of time large masses of dissolved calcareous rock,* 

 and leave behind it on the edges of each tubular hollow a coating of fine 

 clay, which the white chalk contains. 



I have seen tubes precisely similar and from 1 to 5 feet in diameter 

 traversing vertically the upper half of the soft calcareous building-stone, 

 or chalk without flints, constituting St. Peter's Mount, Maestricht. • These 

 hollows are filled with pebbles and clay, derived from overlying beds of 

 gravel, and all terminate downwards like those of Norfolk. I was in- 

 formed that, 6 miles from Maestricht, one of these pipes, 2 feet in diam- 

 eter, was traced downwards to a bed of flattened flints, forming an almost 

 continuous layer in the chalk. Here it terminated abruptly, but a few 

 small root-like prolongations of it were detected immediately below, 

 probably where the dissolving substance had penetrated at some points 

 through openings in the siliceous mass. 



It is not so easy as may at first appear to draw a clear line of distinc- 

 tion between the fixed rocks, or regular strata (rocks in situ or in place), 

 and alluvium. If the bed of a torrent or river be dried up, we call the 

 gravel, sand, and mud left in their channels, or whatever, during floods, 

 they may have scattered over the neighboring plains, alluvium. The 

 very same materials carried into a lake, where they become sorted by 

 water and arranged in more distinct layers, especially if they inclose the 

 remains of plants, shells, or other fossils, are termed regular strata. 



In like manner we may sometimes compare the gravel, sand, and 

 broken shells, strewed along the path of a rapid marine current, with a 

 deposit formed contemporaneously by the discharge of similar materials, 

 year after year, into a deeper and more tranquil part of the sea. In 

 such cases, when we detect marine shells or other organic remains en- 



* See Lyell on Sand-pipes, Ac. Phil. Mag. third series, vol. xv. p. 257, Oct. 1839. 



