ACADEMY F SCIENCES] g^jjj RANGES 239 



work as is here ascribed to it. In the Appalachian region, where the rock floors of the valleys are open for inspec- 

 tion so that the rock structure can be made out, there is no such phenomenon as an anticlinal arch of resistant 

 rock from which one limb has been removed by erosion. . . . The hypothesis of faulting offers so easy an 

 explanation of the unpaired monocline that it has been widely employed by students of Basin range structure; 

 and I am of opinion that the unpaired monocline offers presumptive evidence of a fault. 



3. Cliff Characters. — The ordinary cliff of differential degradation, or the receding cliff, as it is sometimes 

 called, is determined structurally by a strong bed overlying a weak one, and is maintained by weathering and 

 stream erosion. Where drainage lines cross it there are acute re-entrants, and between these re-entrants rounded 

 salients, so that the contour lines are scalloped. Strongly contrasted with this are certain cliffs or steep slopes 

 occuring at many points on the outer face of Basin ranges. For the convenience of a name I will here call them 

 "cut-off" cliffs. Their lower contours, instead of being scalloped, are comparatively direct lines, often straight 

 for considerable distances. Lines of outward drainage, instead of occupying valleys with flaring mouths, issue 

 from comparatively narrow gateways, making but slight interruption in the lower cliff contours. These char- 

 acters are best shown where the rock is resistant and of uniform character. . . . 



In the study of the Basin ranges, I have regarded the cut-off cliff as diagnostic of faulting. Such cliffs 

 may indeed be produced in other ways, but usually the local conditions seem to bar other explanations. 

 Regarding a cut-off cliff as a product of faulting the mode of its origin is conceived as follows: A great fault is 

 assumed to be made by small increments separated by long time intervals. By the first movement a scarp is 

 produced, and each subsequent movement increases its height, but the slowness of the process gives time for 

 the weathering of the scarp and the reduction of its steepness. It also gives time for the continuous adjust- 

 ment of cross-drainage. Streams from the heaved block to the thrown block open gorges in the former and 

 build alluvial fans in the latter, maintaining graded channels. Eventually their gorge walls meet above, 

 reducing the interstream tracts to crested spurs which appear as tho truncated by the # frontal cliff. . . . Cut- 

 off cliffs are also produced by glaciers, but the glacial explanation need not be discussed in this connection. 

 There is in fact but one other mode of origin which seems worthy of consideration, and that is wave erosion, 

 the process suggested by Mr. Spurr. Where waves attack a mountain base their tendency is to straighten the 

 contours by cutting away spurs, and if the process were sufficiently active and prolonged it would be quite 

 competent to produce such cut-off cliffs as are seen in the Great Basin. . . . But the production of the Great 

 Basin cliffs seems altogether too much to ask of the comparatively feeble waves which could be raised in the 

 narrow arms of Tertiary lakes of the Basin. Moreover, the phenomena of Lake Bonneville and Lake Lahontan 

 show that shore cliffs are characteristically associated with terraces or platforms carved from the rock and 

 with embankments of gravel. If the cut-off cliffs of the Basin ranges are wave-wrought the complementary 

 rock platforms and gravel embankments ought somewhere to be discovered. , . . 



4. New Fault Scarps. — It was not until 1876 that certain low scarps, traversing detrital slopes in the Great 

 Basin, were recognized as the surface indications of faults. After their recognition as a distinct physiographic 

 type the positions and directions of numerous dislocations were traced in places where bed rock was and is wholly 

 concealed by valley detritus. . . . The significance of these scarps is greatly enhanced by their association 

 with other features. With few exceptions they follow mountain bases, cutting alluvial slopes close to the 

 boundary between alluvium and bed rock. They occur on the sides of ranges where profound faulting is indi- 

 cated by the transection of folds, by unpaired monoclinal ridges, or by cut-off cliffs. In each case the direction 

 of throw .... is such as to depress the valley or lift the range. 



It is furthermore significant that the fault theory for Basin ranges was founded on other characters, before 

 the recognition of [alluvial] fault scarps, so that their testimony came as a verification; it proved the presence 

 of faults in places where their existence had previously been inferred from less direct evidence. For example, 

 in 1872 a fault was inferred along the East base of the north part of the House range [the Fish Spring range] 

 because an unpaired monocline there presents its scarp to the east, and for similar reason a fault was inferred 

 along the West base of the South part of the same range [House range proper]; and years afterward the theo- 

 retic positions of these faults were found to be marked by low fault scarps, with throws in the theoretic direction. 



In spite of these cogent arguments, the dissenting essay does not appear to have been 

 significantly modified by its author, if he read them. Perhaps it was because of their essen- 

 tially physiographic nature that he regarded them as unconvincing; but surely they must be 

 accepted as compelling by geologists of a more physiographic habit of thought. Two com- 

 ments may be made upon them. The first is that the great advance which they show over 

 the original statement in the Wheeler report proves that the basin-range problem had really 

 been growing and taking on a more and more matured form in Gilbert's mind during the many 

 years in which he had published next to nothing upon it. His report as censor does not appear 

 to be the result of special study or to have cost him any great amount of time; it represents an 

 accumulation of evidence that had been taking shape in his mind as it grew. When the dis- 

 senting manuscript was received, the evidence already formulated in favor of the fault-block 

 theory had only to be written down. The second comment called for by the report is that it 

 adduces no evidence in support of Powell's and Dutton's view that the mountains of Jurassic 



