CHAP, xcv THE PEOPLING OF NEW SOIL 351 



Production of Marsh. 



On the shores of the North Sea, of Kattegat, and of the Baltic Sea, 

 where there is a tide and yet sufficient protection against the violence of 

 the waves, there is deposited at high tide extremely fine mud composed of 

 particles of clay, sand, and humus. In this production of land an impor- 

 tant part is played by vegetation, inasmuch as the mud deposited and the 

 fixed Cyanophyceae (especially Microcoleus Chthonoplastes) find shelter 

 and resting-places between the shoots of, first, associations of Zostera 

 marina l in the deeper water of sand-flats, and thereafter of Salicornia 

 herbacea 2 in less-deep water. Slowly the soil is raised, until eventually 

 the daily high tides can no longer wash completely over it. Then the zone 

 of Salicornia is seized upon by other plants, and gradually, as the land 

 becomes higher and drier, it is occupied by associations of Festuca, of 

 Juncus Gerardi, and others belonging to littoral meadow. 3 In littoral 

 meadow no earthworms live ; but if it be diked and washed out by rain, 

 the raw humus is replaced by mild humus and earthworms occur. 4 In 

 the course of time the soil of littoral meadow will invariably be washed 

 out, and its vegetation will undergo a corresponding change. 



The development of the vegetation clothing reclaimed land at the 

 mouth of the Rhone has been described by Flahault and Comb res. 5 On 

 the low, moist, saline, alluvial land of Camargue Arthrocnemium macro- 

 stachyum establishes itself in the first place. Round it small quantities 

 of sand and organic dust accumulate, and thus raise the soil to a slight 

 extent. Soon the tufts of Arthrocnemium are supplemented by Salicornia 

 fruticosa, Atriplex portulacoides, and Aeluropus litoralis. Fresh material 

 blown by the wind becomes lodged among the prostrate stems of these 

 plants, and gives rise to low hummocks which are 2 or 3 metres in diameter 

 and 10 centimetres in height. Some humus now arises ; rain-water washes 

 out the hummocks ; and other plants, including annuals, establish them- 

 selves. The vegetation may thus become entirely changed in nature, 

 and may include such conifers as Juniperus phoenicea and Pinus Pinea. 



Lowering of Water-level. 



New soil also arises where the level of lake-water sinks so as to lay 

 bare rocks or other substrata that were previously submerged. Malar 

 Lake in Sweden has provided a case of this kind, which has been investi- 

 gated by Callme, Grevillius, and Birger 6 ; in about twenty years this 

 new soil has given birth to forest. 



Volcanic Eruptions. 



Volcanic eruptions may bring into existence plantless tracts. The 

 lava-fields of Iceland were at first devoid of vegetation, and some of them 

 are still extremely poor in plants. Gronlund 7 states that at Myvatn, 

 in the north-east of Iceland, on extensive lava-fields which arose in 1724-9, 

 often there are only crustaceous lichens, including species of Gyrophora and 



1 See pp. 175, 177, 230. * See pp. 225, 230. * See p. 230; also Warming, 

 1890, 1906. P. E. Miiller, 1878. 5 Flahault et Combres, 1894. 



9 Callme, 1887; Grevillius, 1893; Birger, 19060. ' Gronlund, 188-4-90. 



