94 REPORT UNITED STATES GEOLOGICAL SURVEY. 



At some localities it occupies the summits of peaks. When found in 

 this position, exposed to the force of {itmospheric erosive agents, it 

 weathers in exceedingly rugged and often picturesque forms. Eents 

 and cracks traverse the strata vertically, widening locally into caves 

 and almost complete arches. Allowing the widening of such rents, 

 parallel to each other, to continue, we find that the result is a series of 

 sharp pinnacles, ornamenting ridges and slopes of the mountain. Ver- 

 tical precipices denote the cleavage, and readily accomplished disinte- 

 gration of many portions. Frequently, underlying soft strata are 

 removed by the action of flowing water, glaciers or frost, and the super- 

 incumbent ones "drop" down, causing the formation of vertical clift's 

 that have a general resemblance to an amphitheatre. 



Should the conglomerate be stratified, its constitution be sufficiently 

 hard to admit of distinguishing the strata, and they have assumed an 

 inclined position, we will find that the results produced by erosion agree 

 closely with those that may be observed in a coarse grained sandstone 

 under similar conditions. 



Sandstones can be appropriately divided into two main groups, heav- 

 ily bedded and tbinly bedded, each of which may consist of three vari- 

 eties, quartzitic, calcareous, and argillaceous. Of these, the first and 

 third are by far more numerous than the second. When in their normal 

 position, i. e., horizontally stratified, all sandstones show similar general 

 results produced by weathering and erosion. Massive beds in that case 

 present comparatively steep slopes, if quartzitic, rounded and more gen- 

 tle if argillaceous. This is evident from the physical character of the 

 rock. Sandstone is essentially a conglomerate, or vice versa, only that 

 the bowlders, &c., it contains are of very diminutive size. Quartz, car- 

 bonate of lime, and clay form the cements that bind the small grains of 

 sand together. The more loosely they are combined, therefore, the 

 more readily will the rock yield to disintegrating agents. If a series 

 of sandstone strata is not homogeneous, i. c, most of the strata hard, 

 others soft, and the reverse, we will find, j^rovided we have normal strat- 

 ification, that "benches" are formed. With the decreasing thickness of 

 the sandstone beds the outlines of hills or valleys formed change from 

 comparatively abrupt to gentle ones, the summits of hills, rarely mount- 

 ains, are rounded, their ridges obtuse or obliterated, and their slopes 

 curved. With an increase of clay in the rock we observe the same 

 effect. 



Into horizontal beds of this group flowing water cuts valleys or canons 

 of regular shape. Dependent upon the hardness, &c., these may have 

 steep or gentle slopes, both of which will be very similar to each other. 

 Vegetation is, in this instance, a very destructive agent, changing rap- 

 idly the minor details of any exposed surface. By disintegrating the 

 sandstone it produces, mechanically, soil, which in turn is frequently 

 removed, showing the rocks to which it owes its present existence in 

 changed and modified forms. Avalanchial and water-worn drift assume 

 shapes that correspond to the physical nature of the strata from which 

 it is derived. Angular bowlders or round cobble-stones denote a hard, 

 strongly resisting rock. If these former are rounded, the latter flat and 

 of oval or round form, we have a soft sandstone. 



At many localities the vertical or approximately vertical cleavages in 

 sandstone strata can be observed. These give rise to vertical or very 

 steep blufls, by the separation of some portions from the main body of 

 the beds. 



Wherever the strata are not in their normal position, either inclined 

 or standing on edge, the character they exhibit differs somewhat from 



