ESBLicH] SEDIMENTARY ROCKS. 95 



that above given. Again, it is of importance whether the sandstone is 

 a hard or soft one, massively or thinly bedded. As a role, all sand- 

 stones, if their strata are inclined, form hills that show a gentle slope 

 with the dip, a steeper one opposite. Rounded forms prevail, unless, 

 indeed, we have one of those cases where sandstone merely forms the 

 capping of a bluff, overlying more readily-disintegrated rocks. Should 

 the strata stand on end, then they will generally project prominently in 

 narrow ledges, represented sometimes only by isolated "sentinels" that 

 indicate the strike of the stratum. The harder portions, in such a case, 

 will resist eroding influences and remain, while the softer ones gradu- 

 ally disappear, adding, by their absence, to the prominence of the 

 former. 



Attrition by sand which is carried with considerable velocity and 

 force by the wind is a well known agent of erosion that can demon- 

 strate its influence in a region containing sandstones. The results pro- 

 duced by this species of erosion are manifested in the minor details 

 mainly, in rare instances only having much influence upon the general 

 configuration of a mountain or hill. 



Argillaceous sandstone gradually merges into shales and marls, so 

 imperceptibly, sometimes, that it is often a matter of doubt whether we 

 should term certain beds shaly sandstones or arenaceous shales. So, 

 too, do the products of atmospheric and aqueous erosion blend into 

 each other. 



Shales and marls, interstratified with sandstones, are productive of 

 forms that depend in a great measure upon the character of the latter. 

 If they are hard, weathering but slowly, we will generally find that 

 more or less gently rounded hills are the result, which contain nume- 

 rous " steps" or benches, each one the result of the greater resistance 

 shown by the sandstone. If this latter is argillaceous, however, and 

 its beds not too thick, the slopes of hills and bluffs will usually be 

 steep, showing an evenly-sloping angle. In that case, a stratum of 

 sandstone most frequently acts as a protecting cap, preserving the 

 underlying shales from further erosion. Vertical cleavage - planes 

 throughout the series of strata produce local precipitous slopes. 



Water cuts deeply into the members of this group, producing nar- 

 row, steep gullies and ravines. As a rule, their sides are very even, 

 aflbrding a symmetrical profile on a transverse section. Banks of sand- 

 stone or dolomitio beds form benches, running along with great regu- 

 larity, broken where the facility for so doing was afforded by already- 

 existing cleavage fractures or faulted displacements. 



Homogeneous shales and marls, i. e., without interstrata of either 

 sandstones or dolomites, assume very characteristic shapes. They erode 

 in regular forms, representing, en miniature, the slopes of mountain 

 ranges and ridges. Wherever no protecting cap of sandstone occurs, 

 or where it has gradually been carried off, we can observe in the carving 

 of the shales steep slopes, regular sides, and a disposition of spurs and 

 ravines that would admirably correspond to a model range. Differences 

 of color in that case produce very beautiful effects. Never-ceasing 

 denudation slowly but surely levels these small ridges in the course of 

 time. Wherever the shale-beds have been changed from their normal 

 horizotital position they give rise to the formation of valleys. This is 

 more particularly the case if they are over and underlaid by harder series 

 of strata. The lesser resistance of the shales to erosive agents allows 

 them to be removed more rapidly than the neighboring groups, and par- 

 ticularly flowing water shows its activity. Lateral valleys with steep 

 sides, little more, often, than ravines and gorges, enter the main one, and 



