PRE-CAMBRIAN AND TERTIARY MOVEMENTS. 57 



animation, its maximum throw was on the line of the greatest displacement 

 by faulting during the Tertiary movement. It broke into branches and 

 diminished in throw towards each end of the present Butte fault. This is 

 not unexpected, as the Tertiary break was undoubtedly over and along the 

 line of the least resistance below. It roughly duplicated the old line of 

 faulting, only reversing the direction of the movement about 2,000 feet along 

 the greater part of the fault. That the rocks of the Chuar terrane are still 

 displaced from 400 to 2,000 feet, in relation to well-marked Algonkian strata, 

 proves the profound character of the pre-Tonto fault. The movement pro- 

 bably occurred during the progress of the elevation of the Keweenawan 

 continent, and when the Archean (?) and the 12,000 feet of Algonkian strata 

 now unconformably underlying the Cambrian Tonto sandstone, were brought 

 to the surface; the agents of erosion planed off the raised and faulted strata, 

 and not until the remainder of the Paleozoic series, the Mesozoic, and much 

 of the Tertiary were deposited, did any known movement occur on the line 

 of the fault, except to form a slight monoclinal fold at or near the close of 

 the deposition of the sediments of the Paleozoic. (See figs. 11 and 12.) 



The Tertiary Movement. — This has been largely described in giving the 

 details of the various sections, more especially those of section 4, where it is 

 stated that the eastward throw of the combined fold and fault is fully 2,700 

 feet. That the latter movement took place in Tertiary time has been well 

 established by Capt. C. E. Dutton, in his study of the High Plateaus and 

 the Tertiary History of the Grand Canon. In the following description of 

 the flexing or upturning of the strata on the line of the fault, many descrip- 

 tive details will be found that otherwise would be referred to under this 

 heading. 



Flexing of Strata on the Line of the Fault. — The area of pre-Cambrian 

 strata exposed to view by erosion is limited, but from it we learn something 

 of the general geologic structure of the pre-Cambrian surface and the con- 

 ditions under which the flexing of the strata occurred in the vicinity of the 

 Butte fault. 



Eight miles southwest of the southern branches of the fault, the Archean (?) 

 or older Algonkian rocks appear. Here the plane of their upper surface, 

 over which the strata of the Grand Canon group were deposited conforma- 

 bly* and probably horizontally, strikes N. 35° W. and dips 10° to the K E. 

 The strata above partake of this strike and dip, and, with the exception of 

 a broad undulation four miles to the northeast that forms a synclinal and 

 anticlinal. This continues up to the vicinity of the fault. Continuing north- 

 west of the immediate proximity to the fault, the general strike is west and 

 northwest with a dip to the north and northeast, as far as the summit of the 

 pre-Tonto groups at Nun-ko-weap butte, on the divide between Nun-ko-weap 



" * Not conformably to the strata of the Areheau (?), as the greatest unconformity prevails in this 

 respect. 



VIII— Bull. Geol.'Soc. Am., Vol. 1, 1889. 



