246 GROVE KARL GILBERT— DAVIS [MMoiM [vol!xx£ 



buttes of White valley testify to its essential shallowness. . . . The wave theory is inadequate. I doubt if 

 there is a shore cliff 3000 feet high — certainly there is none on the borders of a lake or land-locked bay. 



This conclusion was fortified when the southern half of the range was examined. The 

 highest limestones of the range — K in Gilbert's sections and diagrams — occupy considerable 

 areas of its eastern slope; as seen from the west they are visible in small force at the northern 

 end of the range and on one summit, Swasey Peak, near the mid-length of the crest; but in the 

 southernmost 10 miles of the range they form bold cliffs which cap the west-facing escarpment 

 as it gradually descends to its end. As the whole body of strata in the south has a gentle dip 

 to the southeast, the shales between the resistant capping limestones and the resistant under- 

 lying strata determine an oblique depression — Rainbow Valley — trending northeast-southwest 

 into the range. Gilbert was much impressed with the features of the range front on finding 

 that the resistant basal strata seen along the middle of the range, the weak intermediate shales 

 that obliquely descend to its southern part, and the resistant capping limestones that slant 

 down to its southern end, are all cut off at the same gently sinuous line along the western base. 

 The gradual loss of height by the capping limestones as they decline to the end of the range 

 was repeatedly noted as explicable only by faulting of a comparatively modem date. It is 

 indeed difficult to see how any other explanation can be entertained. 



VERTICAL UPLIFT OR HORIZONTAL EXTENSION 



Had the notes of the summer of 1901 been expanded in a written report it is probable 



that mention would there have been made of a new interpretation not recorded in the field, 



i although the facts and inferences that warrant the record were written down clearly enough. 



It will be recalled that the original announcement of Gilbert's basin-range theory, quoted in 



an earlier section, included the statement: 



The movements of the strata by which the ridges [ranges] have been produced have been in chief part 

 vertical along planes of fracture, and have not involved great horizontal compression. . . . The forces . . . 

 have manifested themselves at the surface as simple agents of uplift, acting in vertical or nearly vertical planes. 



It should also be understood that the notes of 1901 make occasional reference to the "uplift" 

 of the mountain blocks. But if the gently sloping spur-end facets of the Wasatch truly repre- 

 sent the fault surface on which that mountain block was displaced, and not landslide surfaces 

 of later date and of much less inclination than the fault surface, and if the Wasatch fault thus 

 determined is typical of the mountain-block faults elsewhere in the Great Basin, it would seem 

 as if the forces by which the faults were produced could not be any longer described as "simple 

 agents of uplift, acting in vertical or nearly vertical planes," but rather as peculiar agents of 

 extensional uplift, acting on sloping planes and causing a marked broadening of the region of 

 their operations. The mechanical interpretation of such extensional uplifts might have led to 

 views as novel as those expressed in the explanation of the process of laccolithic intrusion. 



A brief reference to this new interpretation is foimd in one of Gilbert's latest essays, m 

 which attention is given, among other matters, to the depth at which subcrustal movement 

 takes place in the reestablishment of an isostatic equilibrium after it has been disturbed. The 

 case of compression is first considered : 



In some regions, such as the Appalachian, overthrusts and folds testify to great reduction in the horizontal 

 extent of rocks at the surface, the reduction having been accomplished in a small fraction of geologic time. 

 If the subjacent portion of the nucleus had been correspondingly forced into narrower space there would have 

 resulted an enormous mountain range, but the actual uprising was of moderate amount. Plausible explanations 

 of the phenomena necessarily include horizontal movements of the upper rocks without corresponding move- 

 ments of the nucleus and thereby imply mobility of an intervening layer. 



The case of extension is then taken up : 



In certain block-mountain districts of the West the master faults are antithetic in type to the overthrust 

 and demonstrate pronounced extension of the upper part of the crust. The nucleal tract beneath could not 

 share in this extension without creating an enormous depression, which does not exist; and the interpretation 

 of the phenomena involves horizontal shear in material more mobile than the visible upper rocks. 6 



4 Interpretation of anomalies of gravity. U. S. Oeol. Survey, Prof. Paper 85-C, 1913; see p. 35. 



