FOOTHILLS STRUCTURE IN NORTHERN COLORADO 733 



This westward travel of 8,000 feet, due to the fault, will be 

 augmented considerably by the change in the character of the 

 monoclinal fold. As has been shown above, the monocline north 

 and south of Golden is normal, gradually steeping and eventually 

 overturning as it approaches Golden. The natural dip of the 

 formations in the normal parts would, therefore, carry their out- 

 crops far to the east of the steeper and overturned parts of the 

 monocline, which, added to the effect of the faulting, is undoubtedly 

 sufficient to account for the present situation of the various forma- 

 tions. 



In connection with the writer's interpretation the following 

 statement from Marvine 1 referring to the condition at Golden is of 

 interest: "Some of the facts at hand indicate that a peculiar 

 fault, depending on the nature of the sharp fold, and possibly 

 connected with the lava near by, may have caused the present 

 appearance." 



The true nature of the fault was not realized by Marvine, for 

 he also states "this may be caused .... by a fault which has 

 pushed the higher portion of the series westward over the upturned 

 edges of the lower portion, thus concealing much of the latter." 

 Such a displacement is, however, incompatible with the data at 

 hand and was hence rejected by later workers. 



It is also of interest to note that Lee and Richardson 2 appear 

 to doubt the existence of local unconformities in the foothill region. 

 Dr. Patton also states in personal conversation that many of 

 the phenomena observed by him in the foothills appear to be 

 incompatible with the arch hypothesis. 



The Boulder area. — The same structural peculiarities are 

 shown at Boulder as at Golden, but not developed to the same 

 remarkable degree and differing in some minor detail. Eldridge, 3 

 and subsequently Fenneman, 4 studied this area in detail and 

 developed an explanation in every respect similar to that advanced 

 for the conditions at Golden (Fig. 9). Elsewhere 5 the writer 



1 Hay 'den Survey, VII (1873), 137, 138. 



2 See Lee, op. cit., p. 32; Richardson, U.S. Geol. Survey, Folio 198, p. 11. 



3 U.S. Geol. Survey, Monographs, XXVII, 105-14. 



4 U.S. Geol. Survey Bulletin 265, pp. 54-66. 



5 Colo. School of Mines Quarterly, April, 191 7. 



