ACADEM* OP SCIENCES] BA g IN RANGES 243 



Many instructive items are later recorded concerning the individual ranges to the west- 

 ward. The Oquirrh Range "shows an anticline cut off obliquely by the Cedar Valley front," 

 in which the harder strata stand up as ridges, while the weaker ones are worn down in reentrants 

 and invaded by the alluvium of the "valley" or intermont plain. 



In one of the reentrants . . . part of the alluvium is a veneer over degraded r. i. p. [rock in place]. This 

 would seem to indicate that the range has long stood without uplift. The growth of the outer plain has raised 

 the base level, thus causing the alluvial area to encroach, and in an intermediate zone the streams have been 

 so long held at one height that they have planed the rock to their grade profiles. Naturally, with this condition 

 there are no fault scarps. 



This passage is significant as being one of the most explicit statements anywhere made by 

 Gilbert with regard to the effects of time upon the form of a fault-block mountain front. Mani- 

 festly, with still longer time, even the hard-rock ridges would be worn back from the fault line, 

 and the physiographic evidence of the faulting would be greatly weakened or lost. 



Another novel point is soon noted. The Oquirrh Eange was seen to have on its western 

 side a sloping surface that had been planed independent of its structure, and that appeared to 

 descend under the valley alluvium. This appears to be the first instance in which Gilbert 

 recognized a prefaulting surface of degradation as such, thus verifying the views of Powell and 

 Dutton as to a long cycle of erosion after the period of deformation by which the ancient moun- 

 tains of the region were produced, and before the period of block faulting which gave rise to 

 the existing ranges. Another old surface of the same kind was seen traversing the synclinal 

 structure of the Lake Range at a high level ; and here the field notes explicitly argue the case to 

 its legitimate conclusion: 



This plain traverses the structure and is a most remarkable survival of a condition before the creation of 

 the faces of the range. It would seem a fair inference (1) that the synclinal structure is older than the summit 

 plain, (2) that the production of the range (by uplift) is more recent than the summit plain, .'. (3) that the 

 uplift is independent of the synclinal folding. That this range should have been ^developed by erosion of sur- 

 rounding [structures?] without obliteration of the summit plain is incredible .'. (by elim.) the range is due to 

 local uplift between faults (weak argument?). 



This is, as far as can be learned, Gilbert's first written statement of the argument for a 

 two-cycle history of the mountains; and as such the doubt parenthetically expressed at its close 

 is not unnatural. Would that the geological world might have had explicit statement of bis 

 later and matured opinion upon this most significant matter. But what that opinion would 

 have been is clearly enough indicated on later pages. The Aqui Range is described as having "a 

 high inclined table, a bit of old peneplain uplifted unequally?" Here the final interrogation 

 mark shows that an open mind was still preserved. Doubt is again given its rightful expression 

 in a note on the Stansbury Range, where it is suggested that certain high-level remnants of an 

 old topography might possibly survive after the erosion of the broad intermont valley; but the 

 antidote to the doubt immediately follows: 



The degradation of the valley by ordinary processes would take so long that the coincident erosion of the 

 mountain would obliterate the old topography. 



When the House Range was finally reached, the doubts vanish and a definite conclusion 

 was reached, as will be told below. 



Other ranges appear, like the Oquirrh, to have long stood still since their last uplift. The 

 Cedar is well described as "a stagnant range draining to a stagnant valley. " It has " no scarps; 

 no trace of fault lines. This is an old worn-down range. " The Simpson Range shows truncated 

 structures on both sides; it is a rather narrow, boldfaced range, but its marginal faults are not 

 continuously traceable. The Dugway is described as a "static range "; a fault, marked by trun- 

 cated structures but not by scarps, was traced for 8 miles along its southwestern face and appeared 

 to extend farther. "In its known extent, the fault [which is 'a mountain making fault, separat- 

 ing the mt. massif from rock waste '] truncated a syncline of great size, bringing in succession 

 to the springing line of the mt. several thousand feet of strata. Its line is definitely a mountain 

 front and at only one point does an important graded plain encroach upon the mountain mass. " 

 The Deep Creek Mountains give the impression of being "primarily an uplifted mass, the 

 bounding fault being sharply marked out by the line between alluvium and rock. " 



