160 Mevieivs. ^Jan., 



have formed unhealthy deltas of the river, has been utilized, and now 

 makes the coast more healthy. The quantity of sediment thus made 

 available artificially, is estimated at not less than twelve millions of 

 cubic yards per annum. 



Among the larger phenomena of nature capable of being brought 

 under human influence, the checking of the progress of sand-hills 

 must be regarded as one by no means slight or unimportant. To 

 understand the agency of man in fixing or rendering permanent long 

 lines of dunes on an exposed coast, or in a desert subject to winds 

 blowing persistently from some one quarter, the nature and origin of 

 such heaps of loose material must be considered. They are not by 

 any means always of the same nature. The particles of sand, gene- 

 rally siliceous and rounded, are sometimes of other rock and angular. 

 There are sands of garnet, others of black titaniferous iron, some of 

 fine, others of extremely coarse grains. Most of these heaps contain 

 much water, so that by digging into them to a short depth fresh water 

 is reached. Hence, by a systematic planting of certain grasses, their 

 movements onwards may be checked, and in time they become conso- 

 lidated. Afterwards trees grow upon them, and they undergo little 

 further change. On the 'other hand, by removing the vegetable cover- 

 ing from some of those formerly existing on the coasts of Flanders, it is 

 not unlikely that the dunes have once more been exposed to the attack 

 of winds. As they often serve as barriers against the sea, it is easy to 

 contemplate the result, should they by any carelessness be allowed 

 to be set in motion. Very serious results of this kind have already 

 taken place, among which may be quoted the irruption of the sea into 

 the fresh-water lagoon of the Lijmfjord, in Jutland, in 1825. This 

 result, which is expressly ascribed to the " mismanagement of the 

 dunes," occasioned changes so remarkable, as to be well worthy of 

 record, and in the highest degree instructive to the geologist. The 

 lagoon was famous for its abundant fisheries. Millions of fresh-water 

 fish were thrown on shore, partly dead and partly dying, and were 

 carted off by the people. A few revived, and still frequent the shores 

 at the mouth of the brooks. The eel has adapted itself to the change, 

 but to the rest the salt has been fatal. It is more than probable that 

 the sand washed in by the irruption covers in many places a layer of 

 dead fish. 



At the time of the accident, also, the bottom of the Lijmfjord was 

 covered with a vigorous growth of aquatic plants, belonging both to 

 fresh and salt-water, especially Zostera marina. This totally dis- 

 appeared after the accident, and, in some instances, was buried under 

 sand ; but even where there was no sand the Zostera, though a marine 

 plant, was destroyed. It is certain, that at some former period there 

 had been a communication between the Lijmfjord and the German 

 Ocean ; so that if we could get a clean section through its bottom, we 

 should find beds of Ostrea edulis and Cardium edule, covered up by a 

 layer of sand, with Zostera and fresh« water fish ; and this again covered 

 with sand and beds of Mytilus edulis. If the new channel should be 

 closed, which it might easily be, the lagoon would be converted first 

 into brackish and then into fresh water, and there would be a further 



