ENDLICH ON EROSION IN COLORADO. 851 



Tbey might be characterized, perhaps, as unequal impreguatiou by the 

 cementing material. Within the group we have been discussing they 

 form a distinguishing feature, although uot found occurring absolutely 

 uniformly throughout its entire horizontal and vertical extent. 



POLE CREEK. 



Pole Creek flows southward into the Kio Grande, which it joins at 

 about west longitude 107° 30' and north latitude 37° 45'. Its course, 

 just before the junction, lies through a narrow, grassy valley. Within 

 this may be found small local accumulations of trachytic tuffs. On the 

 east side of the creek, about 4 miles from the river, a very curious group 

 of eroded rocks occurs. They are composed of light tuffs, more or less 

 firmly cemented. Located immediately upon the bank of the stream, 

 tbey rise abruptly from 12 to 30 feet above the surrounding soil. K"o 

 connection, above the surface, is maintained with any other outcrops of 

 the same material. Owing to a change in the character of the feld- 

 spathic cement, the eroded rocks have assumed most fantastic shapes. 

 A ready imagination can soon recognize in them a venerable exhorter, 

 located within a pulpit, and an appreciative audience of eight or ten 

 persons, either seated or standing in front of him. Were it not for the 

 incongruity, the attempted portrayal of dress might lead the observer to 

 picture to himself a diminutive congregation of devout Knickerbockers. 

 Their stately repose and dignified bearing scarcely disturb the resem- 

 blance. 



It may here be stated that not unfrequently the trachytic tuffs of vari- 

 ous localities show a tendency to weathering in statuesque forms. Often 

 differences can be observed in successive layers ; and again, the admix- 

 ture of quartzose matter will be productive of similar results. In the 

 process of their formation, they are analogous to the sandstones above 

 discussed. Dependent upon the composition, however, is the, effect 

 which sand-blast will have upon them. If the material is yielding — not 

 brittle — then the transportation thereof will be much impeded. 



Besides these localities, there are others in Colorado exhibiting simi- 

 lar features. Textural variations in sandstones, belonging to the Tri- 

 assic and Cretaceous formations, are productive of forms that may be 

 classed as statuesque. Taking into consideration, however, the occur- 

 rences best known, we may say that we shall not invariably expect to 

 find such products of erosion exhibited in more than the two groups 

 above mentioned — in the Upper Green Eiver and in the lowest trachytic 

 series. Others will more properly find their place in the class of 

 " isolated forms". 



MUEAL FOEMS. 



We may appropriately distinguish two groups of mural forms: those 

 resulting from partial removal of continuous series of deposits, and those 

 primarily produced by the intrusion of foreign material within the limits 



