864 BULLETIN UNITED STATES GEOLOGICAL SUKVEY. 



the Upper Cretaceous sandstones, and among those belonging to the 

 lignitic series, this is especially noticeable. Although occurring com- 

 paratively frequently, the phenomenon cannot be regarded as a char- 

 acteristic of either of these groups. In a general way, this feature 

 is comparable to the irregular density of the sandstones of the White 

 Eiver region. As soon, however, as this irregularity assumes the 

 extreme form of concretions, we can no longer expect that great variety 

 of fantastic jSgures there exhibited, because concretionary inclusions 

 are usually shaped after the same general type. 



Before closing the discussion of erosive products, I desire to point 

 out one feature of iluviatile and pluvial erosion that is as instructive as 

 it is beautiful, the carving of uniformly homogeneous deposits. In 

 Colorado, ample opportunity is afforded to study this interesting phe- 

 nomenon. More, perhaps, than by any other geological group, it is 

 exhibited by the soft shales, comparatively free from sand, of the Creta- 

 ceous formation. Frequently may be found bluffs or ridges the sides 

 of which present a most typical miniature arrangement of hills, valleys, 

 mountains, and caiions. What is here accomplished in a comparatively 

 short time on so small a scale, nature's power has successfully completed 

 in successive ages on a scale incomparably greater. Time and the 

 never-ceasing activity of erosive influence produce results that at 

 present fill us with astonishment and admiration. Changing from day 

 to day, in a degree imperceptibly small to us, geological periods have 

 been required to produce what we now see. Nothing, perhaps, 

 expresses more aptly the lesson taught by observing the effects of 

 erosion than the old Eoman verse : 



" Gutta cavat lapidem non vi, 

 Sed 8i3epe cadendo." 



