i 



i-Aix.] GEOLOGI':' STRUCTURE. 33 



stress. It is perhaps due to the presence of a slight horizontal ele- 

 ment in the latter. 



The fault planes are usually vertical. The Rosiclare vein, occur- 

 ring- along such a plane, has been mined to a depth of 800 feet, and 

 in that depth is so nearly vertical that the shaft is still within the 

 vein. The P^nirview vein has an average dip to the west of 79° 30'. 

 The Hamp vein dips S. G8°. The Empire vein dips PI 73°. Even 

 these departures from tlie vei'tical seem, on the whole, to be excep- 

 tional, and the total etfect of the faulting has been to divide the 

 nearly horizontal rocks into a series of polygonal blocks, which 

 have been raised or lowered with reference to one another, but with 

 very little horizontal displacement. This type of structure, while 

 perhaps common, has been but little discussed. Mr. G. K. Gilbert 

 some years ago pictured " it, and Powell gave it the name '' diverse 

 displacement." It is, as he remarks, the Kaibab structure on a small 

 scale. Recently Prof. W. II. Ilobbs has called attention '^ to faulting 

 somewhat similar in character; and in the Globe district of Arizona 

 Mr. P^. L. Ransome has described an intricate system of small normal 

 faults, apparently very much like these.'' In the Columbia folio,'' 

 Hayes and UJrich have mapped one l)lock of strata, evidently 

 bounded b}^ similar fault |)lanes, but with this exception the struc- 

 ture has not been described from the Mississippi Valley- Its occur- 

 rence here is quite in keeping with the general j^eculiarities of the 

 district, which set it off from the remaining portion of the geologic 

 province within which it occurs. 



QUARTZ rrE REEFS. 



The fault planes are commonly marked by the development paral- 

 lel with them of closely spaced fractures. A'^liere these affect sand- 

 stones and the latter have been changed to (juartzite by the addition 

 of silica, reefs or ridges of quartzite, not unlike dikes in topograjDhic 

 expression, are formed. These afford the most obvious means of 

 recognizing fault planes. The juxtaposition of rocks of diverse 

 lithologic character, such as sandstone and limestone, is also indica- 

 tive of faulting, though occasionallj'^ the outcropping edge of a lime- 

 stone interbedcled with sandstone and shale becomes deceptive. 



DIP. 



Near fault planes the rocks often dip at a high angle, and this 

 seems to be especially true in the western part of the district, where 



o Powell, Geology of the Uinta Mountains, 1876, figs. 4 and 5, pp. 16-17. 

 ^ Twenty -first Ann. Rept. U. S. Geol. Survey, pt. 3, pp. 19-162. 



" Geology of the Globe copper distiict : I'rof. I'aper U. S. Geol. Survey No. 12, 1903, pp. 

 07-106. 



" Geologic Atlas U. S., folio 95, U. S. Geol. Survey. 



Bull. 255—05 M 3 



