860 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. 



which the opening extends. In limestones not iinfrequently the exist- 

 ence of an extensive fissure will eventually result in the formation of an 

 arch. This is due, in a great measure, to chemical action. Instances 

 are on record where hills are traversed by a narrow natural tunnel in 

 limestone. Genetically considered, this is an arch, but to the popular 

 mind does not present itself as such. 



IN TEACnYTIC CONGLGMEEATE. 



The scenery on South Eiver has been described in previous pages^ 

 and allusion has been made to the arches occurring there. It will be 

 remembered that high, narrow walls of conglomeritic material are pro- 

 jected toward the stream from the ridge east of it. In these walls, 

 niches and arches are found, some of them of surprising regularity. 

 While making our observations there, we counted altogether eleven 

 complete arches, although more may have been hidden out of our sight. 

 Mr. Ehoda was the first one of the party who successfully ran the risk 

 of climbing along ledges on the wall in order to get into one of the 

 arches. To him, therefore, has been dedicated the one represented by an 

 illustration in the Annual Eeport for 1875 (Plate XX, page 158). A 

 •description of this one will answer entirely for all others there ob- 

 served, as in general shape and method of formation they are very 

 nearly alike. Ehoda's arch is probably the most regular one in out- 

 line. A slight leaning toward the eastward somewhat disturbs its 

 symmetry. Located in a high wall, this arch shows ample dimensions. 

 It is about 180 feet high, 150 wide, and the wall containing it has a 

 thickness of GO to 80 feet. Surrounded by monuments, some of which 

 reach a height of more than 200 feet, the entire view from the point 

 where the sketch was made is one of rare beauty. Pine-trees, 30 feet 

 high, at the base of monuments, appear like pigmies by the side of 

 ;these towering forms. 



Some of the niches or arches at this locality were comparatively acces- 

 sible, and it was found that they were eroded into loosely cemented 

 material. Considering that these conglomerates have been deposited 

 by water, we should expect homogenousness laterally, although varia- 

 tions would probably occur vertically. We have, however, in this in- 

 stance a case analogous to that of the sandstones near White Eiver. 

 By a slow process of infiltration, the large mass has either been rendered 

 more compact, or — reversing the proposition — a process of leaching has 

 rendered certain portions very weak. Either of these suggestions may 

 contain the statement of what has really occurred, because, so far as our 

 observations extend, we find that the physical constitution of the mate- 

 rial in which niches and arches occur is such as to render it more easily 

 yielding to erosive action. Taking into consideration the method of 

 deposition of the conglomerates, it seems illogical to assume that this 

 state of affairs existed ever since the time of its formation : we must 

 therefore seek for a cause to explain the phenomenon. 



