724 VICTOR ZIEGLER 



Montana (Pierre, Fox Hills) 



i. Pierre completely disappears from bottom upward. Absent for 

 distance of about 4! miles. Upper part of Fox Hills, only present 

 at Golden. 



2. Conformable in dip and strike to Laramie. Shows a discrepancy 

 of i5°-2o° in strike with older rocks. 



3. Steepest dips on area at Golden — 90 and overturned (8o° W). 

 Laramie 



1. Broad, sweeping curve by which it is gradually carried to the west- 

 ward until at Golden it lies within 4,500 feet of the Archean. 



2. No thinning. 



Eldridge's Hypothesis op an "Arch" at Golden 



The abnormal conditions tabulated above are all explained as the result of 

 a series of unconformities at the horizons where these occur. 



There is postulated a headland of anticlinal structure with 

 axis perpendicular to the present trend of the foothills. This 

 has been named the "Golden Arch" (Fig. 3). No attempt will 

 be made to discuss the so-called "arch hypothesis" in detail. For 

 such, the reader is referred to previous publications. 1 It will be 

 noticed that the lines along which certain formations disappear 

 are roughly parallel to the general strike of the foothills formations; 

 and that as we approach Golden the formation to the west of such 

 a line — that is, the older — disappears from the top down; while 

 the formation to the east — that is, the younger — disappears from 

 the bottom up. In the "arch hypothesis" such disappearance is 

 explained as the result of the erosion of the older formations from 

 the top of the rising headland, followed by a subsidence and a 

 gradual overlapping of the younger formations against the sides 

 and eventually over the top of this arch. The older formations 

 are missing because of erosion; the younger, because of non- 

 deposition. 



Comments on the Arch Hypothesis: A number of points 

 brought out in a former discussion as well as some additional data 

 from field work seem to show serious weakness in the arch hypothe- 

 sis. These points are in part facts described in detail by Eldridge 

 and summarized above, in part facts discovered since, or at least 

 not specifically noted by him, and in part certain fundamental 



1 U.S. Geol. Survey, Monographs (1896), XXVII, 91-97. 



