DEPOSIT. 43 



beach-sand thrown up by the waves and finally deposited as dunes by the wind, 

 or in the probable wind formation of loess from water-laid plains. In many 

 cases, however, the action of the two agents is more or less simultaneous. 

 This is especially true of the fluvio-glacial deposits due to the combined action 

 of water and glaciers, and of beaches formed by the action of wave-borne ice. 

 It is pecuHarly characteristic of the deposits formed by ground-waters in sur- 

 face streams, though here we are really dealing with a single agent, as is essen- 

 tially true also in the case of snow-drifts due to wind. As to volcanoes, 

 eruptive activity is the one agent concerned in lava-flows and cinder-cones, but 

 this is combined with wind to effect the transport and deposit of volcanic dust. 



Manner of deposit. — This depends upon the kind and natmre of the agent 

 and upon the kmd of material. Ground-waters carry material in the finest 

 condition, since it is in solution, and hence such deposits as sinter and traver- 

 tine are the most uniform of all in composition and texture, if certain char- 

 acteristic irregularities of surface are disregarded. Such deposits owe their 

 uniformity and density, moreover, to the fact that the water contains cement- 

 ing material alone, so to speak, while in the case of surface-water the solid 

 particles are in much larger quantity than the material in solution. Winds 

 also carry particles of a small range of size, and the resulting deposits are 

 essentially homogeneous. As a consequence of the lack of cementing material 

 in solution, dunes, sand-hills, masses of volcanic dust, etc., are also character- 

 istically unstable. An exception to this is furnished by loess, though the 

 stabihty here is perhaps due to the later cementing action of absorbed water. 



Water and ice exhibit the widest range in the size of the materials carried 

 and in the amount of cementing action present. This is of course particularly 

 true of glaciers. They show the most striking difference in the sorting of 

 materials, moreover, as is well known. Lateral sorting is practically absent 

 from true glacial deposits, while it is typical of water sediments. Glacial 

 deposits possess much less cohesion in consequence of this fact and of the 

 wider range in the size of particles, but also because of the greater lack of 

 cementing substances. 



The nature of the soUd particles and of the cementing materials is also 

 a determining factor of the hardness of the deposit. While an uncemented 

 deposit is ready for invasion as soon as water conditions warrant, sedimentary 

 rock must first be weathered before it will permit penetration or possess the 

 requisite water-content. Rocks cemented by lime respond most readily to 

 weathering processes, though many exceptions are produced by differences in 

 the amount of cement, quite apart from its nature, and also by pressure and 

 metamorphism. Differences in the material of the particles, as between 

 sand and clay for example, are controlling as to the holard and echard, and are 

 consequently decisive in the ecesis of pioneer migrants. 



Rate and depth of deposit. — ^The rapidity with which a deposit accumulates 

 depends upon the amount of material carried, upon the duration or frequency 

 of the agency, and upon the barrier to movement which effects the deposition 

 of the load. The rate of deposit is of importance in determining the rate at 

 which vegetation is overwhelmed and at which the deposit will reach a point 

 where colonization will be possible. It also affects the reactions of the early 

 stages of succession, as well as the period of each. These are often more 

 directly related to the continuous or intermittent nature of the deposition than 



