92 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



minerals are forced off or loosened, and thus the more rapid erosion by 

 other agents is facilitated. 



Diminishing with the size of the constituent minerals are the charac- 

 teristics above given for coarse-grained rocks of this group. As the 

 former become smaller, the aggregate forms a more compact mass, less 

 assailable by atmospheric influences, water, and sand. Instead of 

 rounded hills we will find steeper mountains, showing sharp ridges 

 leading toward the summit. Abrupt changes of the angles of slope 

 and precipitous edges will denote the more thoroughly resisting ma- 

 terial. While here we find higher mountains generally, coarse-grained 

 masses will rarely occupy any very elevated position, being too readily 

 reduced by disintegration. These features, however, can change if we 

 have before us a stratified or stratoid mass of rocks. In this case the 

 features of mountains and ranges no longer conform to the synopsis 

 above given, but are subject to the same variations that are character- 

 istic of the next following group. 



While the avalanchial drift from coarse-grained granite or kindred 

 rocks generally occurs in more or less rounded bowlders, that of the 

 finer-grained varieties is angular, with sharp edges, and flat instead of 

 rounded sides. If worn by water, the latter will, as a rule, show more 

 regular forms, owing to its greater degree of homogeneousness. 



The second subgroup : Gabbro and schists appear as totally distinct 

 from the preceding, if we except the possible bedded condition of the 

 latter. It is a very rare occurrence to find schists lying horizontally. 

 Generally they are tipped up, more or less steeply inclined, and we have 

 in their exterior character the combined influence of a crystalline aggre- 

 gate and stratified structure. Very rarely are coarse-grained varieties be- 

 longing to this series met with. Usually the component minerals are small 

 and distributed evenly throughout the masses. Acicular crystals lying 

 in one direction tend often to increase and subdivide the distinguishable 

 strata or layers. A predominance either of quartz or hornblende en- 

 ables rocks of this group to resist very successfully the attempted dis- 

 integration. For the same reason growing vegetation will not. pro- 

 duce so extensive a loosening or removal of fragments as it does in a 

 coarse-grained rock. Flowing water selects for destruction the most 

 readily yielding beds, and leaves others either as partially rounded nar- 

 row benches, or broken fragments, denoting the course of the former 

 continuous hard stratum. 



Whenever we find the strata inclining or standing on edge we will 

 observe that they form steep, sharp ridges, and very olten high peaks. 

 (This latter is due, however, not entirely to the resistance of the rock 

 composing them, but to the fact that this group belongs, as a rule, to 

 the metamorphic series, and its members have acquired their elevated 

 position by virtue of directly active forces.) In that case the slope of 

 the mountains i)arallel with the dip of the strata is smooth, while the 

 one exposing their edges is generally exceedingly ragged. Dependent 

 upon the angle of the dip and that of the latter slope is the formation of 

 precipices and overhanging walls. 



Vegetation has more influence in removing portions of these rocks 

 than it exercises with middle or fine-grained rocks of the first subgroup. 

 This is accomplished inasmuch as the growing roots can more readily 

 enter the narrow crevices between single layers or strata (the former 

 of which are frequently exceedingly thin), and can thereby produce a 

 dismemberment of numerous particles. Avalanchial drift of this sub- 

 group shows characteristics that indicate the bedding of the entire mass, 

 and the more or less frequent occurrence of narrow cracks and fissures. 



