September 20, 1901.] 



JSCIENCE. 



457 



McCullocli Williams writes. It would seem as if 

 anyone intending to publish concerning natural 

 history .would at least familiarize himself or 

 herself with the subject. It is discouraging to 

 find a magazine like McClure^s accepting and 

 printing an article of this character. 



John B. Smith. 

 New Beunwsick, September 12, 1901. 



CURRENT NOTES ON PHYSIOGRAPHY. 

 THE RANGES OF THE GREAT BASIN. 



The mountain ranges of the Great basin in 

 Utah and Nevada have been explained, chiefly 

 by Gilbert and Russell, as due to block faulting, 

 but without sufficient statement concerning the 

 form of the region before faulting or of the 

 amount of erosion since faulting. Hence the 

 ranges have sometimes been imagined as pre- 

 senting long, gentle back slopes where the pre- 

 faulting surface has been tilted up, and abrupt 

 frontal cliffs where the fault scarp is revealed ; 

 and in the absence of statement to the contrary 

 it has been sometimes supposed that the faults 

 by which the blocks are limited were determined 

 by ordinary stratigraphic evidence. 



Spurr now offers a new interpretation of these 

 ranges (' Origin and Structure of the Basin 

 Ranges,' Bull. Geol. Soc. Amer., XII., 1901, 217- 

 270, pi. 20-25). Finding mouoclinal structure 

 not persistent, finding much dissection on both 

 slopes of the ranges, and finding no strati- 

 graphic evidence of faulting along the base of 

 the ranges, he discards the theory of block fault- 

 ing and explains the mountains as residuals of 

 a disordered and greatly denuded mass, the in- 

 termont depressions being regai'ded as valleys 

 of erosion produced under a former greater rain- 

 fall and now clogged with waste since a drier 

 climate has set in. 



Attention is here called to the different values 

 given by Spurr to the stratigraphic evidence of 

 faults where both members of the faulted series 

 are seen, and to the physiographic evidence 

 where only one member is visible. Faults de- 

 termined by ordinary stratigraphic evidence are 

 spoken of as ' actually observed ' (266), while 

 faults announced by previous observers on 

 physiographic evidence are altogether rejected, 

 apparently because they are not confirmed by 



stratigraphic proof. As a matter of fact, no 

 faults (meaning thereby surfaces of fracture on 

 which movements of dislocation have taken 

 place) have been actually observed as such in 

 the Great basin ; faulting is there as elsewhere 

 a matter of inference. In the case of faults 

 proved by stratigraphy, the termination of one 

 series of strata against another may be more or 

 less closely observed ; and then instead of be- 

 lieving that both sets of strata were ' made so ' 

 in the beginning, it is reasonably inferred that 

 they both originally had greater extension, that 

 they were brought into their present relations 

 by dislocation, and that the dislocated mass has 

 been carved into its present form by greater or 

 less erosion. This demonstration is commonly 

 accepted as so compulsory and all other expla- 

 nations seem so infinitely improbable, that fault- 

 ing proved by stratigraphic evidence is often 

 treated as if it were a matter of first-hand ob- 

 servation, and given an equal order of verity 

 with the plain facts of strike and dip. 



In the case of faults proved by physiographic 

 evidence, the outcrops of a series of strata in an 

 escarpment or on a mountain side are directly 

 observed; and then instead of believingthat they 

 were made so, it is inferred that the invisible 

 parts of the series have been in some way re- 

 moved. On finding that their removal cannot 

 be reasonably accounted for by erosion alone, 

 the aid of faulting is invoked, a greater or less 

 amount of erosion being supposed to precede 

 and to follow. This argument of course in- 

 volves such a knowledge of the observable facts 

 of structure and form, and such an understand- 

 ing of the processes of erosion and of the forms 

 resulting therefrom, that, while certain forms 

 (such as the Appalachian ridges of Pennsyl- 

 vania) may be reasonably' ascribed to erosion 

 alone acting on a deformed structure, certain 

 other forms (such as the Basin ranges) may as 

 reasonably be held to be beyond production 

 by erosion, without relatively recent faulting. 

 This demonstration of faulting may be just as 

 logical as that based on stratigraphy alone, but 

 it is somewhat more complicated and it is much 

 less commonly employed. 



Now inasmuch as the block faulting of the 

 Basin ranges has been determined in nearly 

 every case by the physiographic method alone, 



