72 THE BUILDING OF AN ISLAND. 



plain we find its upright banks to exhibit sections showing the gravelly debris 

 from the hills, and in some parts we find the watercourse widened, and we can 

 see that new deposits of the same debris are being laid down on its widened 

 bed. On the other (eastern) side of the Ground we find a similar, but aban- 

 doned, watercourse. It is nearly as deep as the western one, but big trees are 

 now growing in it, showing that no heavy current has passed along it for many 

 years. Possibly it may never be opened up again ; at present it serves only to 

 carry off the drainage from the adjacent hillslope and from part of the Ground, 

 which we may notice has a very slight rise in the middle. In earlier ages it is 

 not unlikely that the stream, which, as we have seen, now runs along the west 

 side of the Ground and formerly also the east side, wandered from side to side, 

 bringing down the debris and extending and levelling the little plain to the 

 form in which we now find it. 



The way in which a stream may lower the surface of a plain is well illus- 

 trated in a watercourse which crosses the plain west of the estate Longford. 

 To the south of the highroad it will be seen that the stream is cutting into its 

 east bank, which shows vertical walls of debris that frequently break away and 

 fall into the watercourse in considerable masses, while on the west side there 

 are no such walls, and only at a considerable distance from the stream is there 

 even a sloping bank to indicate where the water has once been. 



In connection with the plains we may consider the case of the lagoons 

 and seaside ponds. Krause Lagoon, on the south side of the island, is a rather 

 extensive area of islets and mudbanks, separated by water channels and ponds 

 connected with each other, and on its western side with the sea. The islets 

 and mudbanks are grown over with the red mangrove {RhizopJiora Mangle) 

 and the water is always more or less muddv. Along the seaward side of the 

 area we find a bank of sea sand of the ordinary character, but along the land 

 edge we find everywhere that the shore and the bottom are covered by clay 

 and fine gravel, which have been carried into the lagoon from the land. It 

 follows that the lagoon is being slowly but surely filled up by drainage from 

 the land. It will, at some future and distant age, become a plain. The streams, 

 which now rush in from time to time, spread out over it and drop their mud 

 all over its bottom ; but the streams of that future day will continue their 

 courses over it, partially cutting into it antl bringing the heavier debris to lie 

 upon it, and thus a plain of the onlinary type of the coast ])lains of our island 

 will be created. 



The two ponds of Southgate and Great Pond are, in a similar manner, be- 

 ing filled up. Their existence seems to imply that after the plains, of which 

 they occupy the lowest i)art, had been formed by the streams from the hills, 

 there was a sinking of the land, producing bays of the sea, that the sea, in the 

 way described in an earlit-r chapter, then formed tongues of sand shutting in 

 the innermost parts of the bays and thus forming salt ponds or lagoons, 

 which have since been gradually filling up. The sand bar in each case is broken 

 through after heavy rains, when the accumulated water in the pond forces an 



