50 THE MODE OP OCCUEEENCE OE EOCKS 



been deposited from aqueous solution or sublimation between 

 the walls of a fissure; and (2) segregation veins, in which the 

 component materials have crystallized or segregated out of the 

 still unconsolidated, pasty, or colloidal rock. It is not always 

 possible to decide to which of the two classes a vein may be- 

 long, but as a rule the mineral (or fissure) veins are separated 

 by sharp and well-defined walls from the country rock, and 

 show a comb or banded structure. The segregation type is less 

 distinctly marked, the vein material being welded to the enclos- 

 ing rock, or seemingly passing into it by gentle gradations. 



The unconsolidated materials, as sands and gravels, occur 

 not only in regularly bedded or stratified forms, but also in 

 hillocks and ridges to which special terms are applied. The 

 loose material washed down the mountain slopes by ephemeral 

 streams, and deposited at the mouth of gorges, may assume the 

 form of ^'a conical mass of low slope descending equally in all 

 directions from the point of issue. ' ' To such forms Gilbert has 

 given the name of alluvial cones. The material of these cones 

 varies in size from the finest powder to angular rocks weighing 

 many tons. It exhibits no regular bedding or stratification, but 

 coarse and fine debris are mingled in endless variety. There is a 

 well-marked gradation, however, to be seen as one travels from 

 the apex of a cone toward its periphery. At the apex it is com- 

 posed mostly of coarse, angular material, with fine silt-like clays 

 filling the interspaces, while toward the periphery the fine ma- 

 terial predominates. An alluvial fan differs in having greater 

 width in proportion to its thickness and in showing signs of 

 stratification. The name talus is given to the accumulations of 

 debris at the foot of rocky cliffs. Such are composed of angular 

 fragments, large and small, which have fallen from the cliffs 

 above. The name dune is given to the rounded hills of wind- 

 blown sand common in arid regions and on windy shores. 

 These are naturally of moderately fine and quite uniformly 

 assorted materials. In form and position they are ever chang- 

 ing, like drifts of snow, but are usually much steeper on the 

 leeward than on the windward sides. The character of the 

 material of which they are composed is most commonly sili- 

 ceous sand. 



The names hame, esher, osar, and horseback are given to ridges 

 and mounds of sand and gravel deposited by the melting ice of 

 the glacial epoch. Drumlin is the name given to the peculiar 



