May, 1904. . GEOLOGY OF WESTERN MEXICO—FARRINGTON. 201 
of the area, the character of which can be judged from the accom- 
panying views, Plates LVII. and LVIII. 
The area is one of erosion forms, the channels between which are 
worn down to a remarkably uniform level, so that prominences seem 
to arise thickly over it as from a floor. The height of these promi- 
nences is from about thirty feet down. As compared with the erosion 
forms so common in the bad lands, those of the Ciudad de Rocas 
merit attention on account of the absence of pinnacles, peaks, ridges, 
and angular outlines in general. . Their outlines are, on the contrary, 
domed and rounded, and exhibit convex and concave curves. On 
closer study the difference in outline appears less than at first sight, 

Fic. 1. Weathering of structural block with talus. After Hicks. 
since, as has been shown by Gilbert,* the profiles of bad land slopes 
also exhibit convex and concave curves when viewed in a large way. 
Convex curves upon eroded areas have been shown by Hicks to be 
due to weathering and concave curves to water sculpture.f The man- 
ner of production of the convex or weather curve is thus described 
by that author: ‘“‘The weathering of structural blocks reduces their 
salient angles, which are attacked from both of the adjacent faces 
at once. At the point x (Fig. 1) the disintegrating forces act with 
twice as great intensity as at b, since the attack comes from two direc- 
tions. The effects are more than twice as great at x, because the 
products of decay are quickly removed, exposing fresh surfaces to 
the attack, while at b they remain to cover and protect the subjacent 
beds. Thus the structural block m1 p is rounded off by weath- 
ering. The new outline abc is composite. The portion d b e 
is a weather curve, convex upward. If weathering alone, without 
the aid of flowing water, has been concerned in the sculpturing pro- 
cess, the talus slopes ad and e c will be structural planes, not curves. 
The structural angle ec p will be determined by the resting angle 

*Report on the Geology of the Henry Mountains, pp. 122, 123. 
{Bull. Geol. Soc. Amer., Vol. 4, pp. 133-146, 1893. 
