44 INITIAL CAUSES. 



to the rate itself. The depth of the deposit is chiefly an effect of the rate and 

 duration, but it also has to do with the area as well, a fact axiomatic of low- 

 lands (plate 6 a). 



Place of deposit. — The place of deposit is critical for two reasons: (1) it 

 controls the water conditions of the new area, and (2) it determines the climatic 

 area and the climax formation in which the new sere will develop. Places of 

 deposit fall into two distinct groups, namely, (1) in water, (2) on land (plate 6). 

 These differ primarily, and sometimes only with respect to the extremeness 

 of conditions as to colonization. Deposits in water must be built up to a 

 level at which submerged plants can ecize before the sere proper begins, a 

 process which is often a matter of centuries and ages. They can be invaded 

 only by water-plants, and the early stages of succession are often very long. 

 Deposits on land, however, can be invaded at once. The physical conditions 

 are necessarily further from the extreme, a wider range of life-forms can enter 

 as pioneers, and the stages of development are usually fewer and shorter. 



Deposit by water is regularly in water, except in such cases as surface wash, 

 but the withdrawal of flood-waters produces what is essentially a deposit on 

 land. Aeolian deposits, on the contrary, are mostly on land, primarily because 

 the material composing them is picked up from beaches and flood-plains by 

 winds blowing from the water area. In the case of dunes, however, they may 

 be carried into lakes, ponds, and swamps, and initiate a sere widely divergent 

 from that on the dunes proper. The course of successional development also 

 depends upon deposition in salt water, fresh water, or alkaline water. Water 

 deposits may be changed into land areas by drainage and elevation, and the 

 land deposits into water deposits in vegetational effect by flooding and subsi- 

 dence. The elevation of water deposits has naturally been a chief initiating 

 cause of the great eoseres of the geolpgical past. Gravity deposits occur with 

 equal readiness and in countless numbers along sea-coasts, lake-shores, and 

 stream-banks, and in all hilly and mountainous regions. Along shores they 

 are in land or water or both; in the case of hills and mountains they are typi- 

 cally land deposits. Glacial deposits produce both land and water areas, 

 though they are first actually laid down in water as the ice melts. The same 

 is true of fluvio-glacial deposits, though these necessarily show more relation- 

 ship to water deposits. 



Distance of transport. — Transported material is deposited at every con- 

 ceivable distance from the place of origin. It may be washed by water or 

 blown by wind into a crack a few millimeters distant, or it may be carried thou- 

 sands of miles and find its resting-place on the bottom of the ocean. Water 

 deposits may be found at the greatest distance from their source, and glacial 

 deposits come next in this respect. The range of wind deposits at the present 

 day is much less, while deposits due to volcanoes, ground water, and gravity 

 are local. Distance naturally effects no sharp distinction between deposits, 

 but it is a factor to be considered, especially in the relation of the new area to 

 migration and to clunax vegetation. From this point of view it is profitable 

 to distinguish (1) deposition in the minor community where the material origi- 

 nated, (2) in other communities of the same climax association, (3). in areas 

 controlled by earlier stages of a sere, but in the same climatic region, (4) in 

 another climatic region, and hence another climax formation. As is at once 

 evident, the point of initiation, the course of development, and the final climax 

 all hinge upon the effect of distance. 



