ruDLicH.] SEDIMENTAEY ROCKS. 93 



la accordance with their resistance to decomposing and disintegrating 

 influences, the bowlders show sharp, jagged edges, and abrupt, trans- 

 verse fractures, besides those parallel to the stratoid arrangement. 



As an incidental agent of erosion, moving ice may be mentioned. Its 

 success, as such, is commensurate with the homogeneousness of the rocks 

 over which it passes, and dependent greatly upon the circumstance 

 whether it takes its course over massive or stratified portions. In the 

 former instance, it will produce a planing, a rounding of the rocks sub- 

 jected to its action, the resuit obtained being forms that have received 

 the appellation ot " roches moutonneesP In a similar manner, though 

 not so decidedly, is the movement of glaciers upon other members of 

 these groups demonstrated. Should they pass over the upturned edges 

 of schistose rocks, they will remove more material from the softer strata 

 than from othe.s, and produce "troughs" in case their course be par- 

 allel to the strike of the strata. In this respect, as well as in the first, 

 the action of ice and flowing water with its accompanying bowlders 

 is very similar. Where, however, not only the base but the walls of <i 

 ravine or caiion are attacked by glaciers, the result is too character- 

 istic to admit of comparison. 



SEDIMENTARY ROCKS. 



The sedimentary rocks have been divided into two groups, those repre- 

 senting merely mechanical deposit, and others necessitating chemical 

 action. 01 these the former is susceptible of subdivision into 



(1.) Conglomerate^ 

 (2.) Sandstone^ 

 (3.) Shale, and 

 (4.) Marl. 



It depends, in conglomerates, upon their physical properties what 

 result atmospheric and other abrading influences will have. Mainly 

 two varieties may be distinguished, soft and hard conglomerates. Of 

 these the former yield very readily, while the latter resist more success- 

 fully. Conglomerates are originally a mechanical deposit of erratic 

 bowlders, of various size, pebbles and sand, cemented either by the lat- 

 ter, by clay, or in rare instances by carbonate of lime or quartz. If 

 sand forms the cement, flowing water, rains, frost, &c., will easily remove 

 it, and the bowlders, at one time held together, will be loosened and 

 carried off. Unless a protecting cap of some kind may prolong the 

 existence of soft conglomeritic masses, they yield in time entirely to 

 destructive agents. Should the conglomerate be a hard one, we will 

 generally find that it contains softer parts, the distribution and shape 

 of which is either irregular or stratoid. These will weather out more 

 rapidly than the remaining portions, and form cavities or gulches, ravines, 

 &c. In that case the hard, remaining rocks will form crags, or irregu- 

 lar masses. As a rule, conglomerates of this character weather in steep 

 slopes. Gradual erosion will carry away more and more of the matrix, 

 until the bowlders contained within it project on the wall or cliff and the 

 latter appear studded with them. Whenever they form creek-beds, we 

 observe that the water rapidly cuts them away, unless, indeed, they be 

 cemented by quartzitic rock or quartz. It is a very rare occurrence 

 that conglomerates are found otherwise than in a subordinate position 

 as regards the structure of a mountain. An exception of this kind may 

 be noted in the Sawatch Eange, where a series of conglomeritic beds, 

 600 to 1,500 feet in thickness, stretches over a very large area of country. 



