202 FIELD COLUMBIAN MusEUM—GEOLOGY, VoL. II. 
of the materials composing the talus, and that again will depend 
upon the size and form of the particles; but in humid regions the talus 
slopes will be quickly molded into water curves. * * * The result- 
ing form a b ¢ will be a rounded rock, a smooth knob, or a round- 
topped hill or mountain, according as the original block was measured 
by inches or leagues.”’ 
While this account may be considered in a general way descrip- 
tive of the process which has been foremost in the production of the 
forms of the Ciudad de Rocas, the manner there has been somewhat 
different, since there is a notable absence of talus in the area. Not 
only is there practically no talus on the slopes of the prominences, 

Fic. 2. Weathering of structural block without talus. 
but it is even lacking on the level floor as well. While a few 
small trees and bushes find sufficient soil for their growth, the 
area is practically treeless, although surrounded by a dense forest. 
Hence the sides and tops of the prominences lack the protection 
postulated by Hicks, and a somewhat different form is produced. 
In general, it may be assumed that this form would be somewhat 
steeper (Fig. 2), since the protective influence of the talus at the sides 
would be lacking. Nevertheless, the tendency to steepness from this 
cause would be somewhat counterbalanced by a similar lack of pro- 
tecting talus at the top, so that the form in any particular case would 
be the resultant of alargenumber of forces. Tlie remarkably complete 
removal of the products of decay from the Ciudad de Rocas indicates 
probably a relatively slow disintegration of the rock as compared 
with the speed at which the sandstones and clays of bad land regions 
break down. This relatively slow disintegration is due partly to 
the texture and composition of the rock, and partly also,probably 
to a scarcity of joints, thus limiting the number of points of attack. 
The detritus formed is thus not greater in amount than can be re- 
moved by local winds, rains, and rills. Another structural or textural 
feature indicated by the convex forms is an increasing hardness of 
