728 VICTOR ZIEGLER 



The accumulative weight of the objections summarized above 

 is so great that the writer unhesitatingly rejects the arch hypothesis 

 as a possible explanation of the major structural features shown 

 at Golden. 1 It is weak in its inherent fundamental assumptions, 

 does not explain the many and peculiar coincidences in the facts 

 observed along the line of the supposed unconformities, and is in 

 several cases directly contradicted by dip and strike observations. 



Proposed Explanation of the Structure at Golden: The geo- 

 logical conditions at Golden, outlined above, are best explained 

 on the basis of extensive faulting practically parallel to the general 

 strike of the formations. It is logical to believe that intense mono- 

 clinal folding of the "S" type shown to be characteristic of this 

 part of the foothills could not be localized along a practically straight 

 line without a decided tendency to form fractures and' faults 

 parallel to the general trend of the fold. On the accompanying 

 geological sketch map of the vicinity of Golden the location of such 

 faults is indicated by heavy lines, and it will be noted that these 

 coincide with the lines of two of the unconformities of Eldridge 

 in his arch hypothesis. The accompanying sections show in detail 

 the faulting as postulated by the writer. In the case of section 

 A- A, the double fault and its effect on the strata in the monoclinal 

 fold is shown by a reconstruction of the latter in its original con- 

 dition preceding each displacement (Figs. 4-7). 



It will be noted that the faults are considered to have steep 

 westward dips. The author has arrived at this conclusion from a 

 study of the relationship of the course of the fault line to the topog- 

 raphy, from a consideration of the character of the monoclinal 

 fold, and the effect of the fault on the displaced rock formations. 

 Ordinarily a thrust fault with dips as steep as indicated would be 

 considered unusual, but we must carry in mind the fact that, in 

 most cases of thrust faulting, lateral pressure is the dominant cause 

 — as, for example, in the southern Appalachians. Here, therefore, 

 thrust faults are as a rule characterized by flat dips. In the case 

 of a monoclinal fold such as this, however, the maximum pressure 



1 Richardson, U.S. Geol. Survey, Folio 198, p. 11. It is of interest to note that 

 both Lee and Richardson appear to doubt the truth of the arch hypothesis. In this 

 connection see Lee, op. cit., p. 32. 



