96 JOSEPH P. IDDINGS 



Numerous parallel joints would aid erosion, if it were once 

 located along a dominant fissure, by permitting the falling in of 

 the sides of the drainage channels. Consequently, it is reason- 

 able to expect that planes of fracture will in some cases become 

 lines of drainage and pronounced erosion, while in other cases 

 they may be disregarded by streams. 



It is also known to every field worker that dislocations in large 

 areas of massive rocks are easily overlooked, and are often dififi- 

 cult to determine when suspected; and further that minor fract- 

 ures are generally neglected in geological field work. It follows 

 from this that much evidence that may exist connecting the 

 location of drainage with rock fractures has not been collected, 

 and much that might be sought, by the very nature of the prob- 

 lem, may not be found, because the bottom of a valley is usually 

 filled with loose material that conceals the rocks through which 

 the valley has been cut. 



A study of a mountainous region such as that lying north 

 ■of the Yellowstone National Park must convince one that fracture 

 systems have had more influence on the location of drainage than 

 is ordinarily supposed, or than can be actually demonstrated, 

 perhaps, by the evidence obtainable from the region in its present 

 condition of rock exposure. 



With regard to the presentation which follows, it is to be 

 remarked that the problem of a possible fracture drainage sys- 

 tem was not in mind when the writer was in the field, and no 

 special search was made for evidence bearing on the question. 

 The argument offered is based on such observations on the struct- 

 ure of the region as were made in the field by Mr. W. H. Weed 

 and the writer, and on the drainage features of the map prepared 

 by the topographers. 



The topographic map which is reproduced is a reduction of 



that of the Livingston Quadrangle, Folio i of the Geologic Atlas 



of the Uiiited States. The geology of the district may be found 



in the folio. In this connection the map of the Three Forks 



Quadrangle Folio 24, and those of the Yellowstone National 



Park, Folio 30, should be studied.^ 



'U. S. Geological Survey, Geologic Alias of the United States, Folio i, Washington 

 1894; Folio 24 and Folio 30, 1896. 



