386 ECOLOGY. 



are slowly but surely playing a part in the changes of soil and in 

 the topography of certain regions. This is very well marked in 

 the region of small ponds, where the bottom slopes gradually out 

 to the deeper water in the center. Striking examples are some- 

 times found where the surface of the country is very broken 

 or hilly with shallow basins intervening. In what are termed 

 morainic regions, the scene of the activity of ancient glaciers, 

 or in the mountainous districts, we have opportunities for study- 

 ing plant formations, which slowly, to be sure, but nevertheless 

 certainly, fill in partly or completely these basins, so that the 

 water is confined to narrow limits, or is entirely replaced by plant 

 remains in various stages of disintegration, upon which a charac- 

 teristic flora appears. 



695. A plant atoll. In the morainic regions of central New 

 York there are some interesting and striking examples of the ef- 

 fects of plants on the topography of small and shallow basins. 

 These formations sometimes take the shape of ' ' atolls, ' ' though 

 plants, and not corals, are the chief agencies in their gradual ev- 

 olution. Fig. 484 is from a photograph of one of these plant atolls 

 about 15 miles from Ithaca, N. Y. , along the line of the E. C. & 

 N. R. R. near a former flag station known as Chicago. The basin 

 here shown is surrounded by three hills, and is formed by the 

 union of their bases, thus forming a pond with no outlet. 



696. Topography of the atoll moor. The entire basin was 

 once a large pond, which has become nearly filled by the growth 

 of a vegetation characteristic of such regions. Now only a small, 

 nearly circular, central, pond remains, while entirely around the 

 edge of the earlier basin is a ditch, in many places with from 

 3o-6ocm. of water. There is a broad zone of land then lying 

 between the central pond and the marginal ditch. Just inside of 

 the ring formed by the ditch is an elevated ring extending all 

 around, which is higher than any other part of the atoll. On a 

 portion of this ring grow certain grasses and carices. The soil for 

 some depth shows a wet peat made up of decaying grasses, carices, 

 and much peat moss (sphagnum). In some places one element 

 seems to predominate, and in other cases another element. On 



