226 



SCIENCE 



[N. S. Vol. XXXVII. No. 9i5 



CRITICAL CRITERIA ON BASIN-RANGE STRUCTURE 



As commonly regarded, basin-range moun- 

 tains constitute an orogenic type by them- 

 selves; novel, isostatic, youthful appearing. 

 The hypothesis of their structure is one of the 

 most brilliant concepts in the history of Amer- 

 ican geology ; at the same time it is one of the 

 most fanciful, as the severe testing of a gen- 

 eration amply proves. Singularly enough, 

 the theory had its birth in a district where 

 even its fundamental form seems to be en- 

 tirely without representation. 



At this day and distance the extension of 

 the hypothesis to all the so-called fault-block 

 mountains of the arid regions appears to be 

 not only too broad a generalization, but quite 

 unfortunate. Although I should not wish to 

 be the first to make so sweeping an assertion 

 as lately was done by Dr. Spurr, that no one 

 has ever seen the fault-lines blocking out the 

 desert ranges, his statement is almost literally 

 true, as all recent critical evidence on the 

 subject fully attests. 



The attractive feature of Gilbert's theory of 

 basin-range structure was of course the strong 

 support it was thought to give to the now 

 famous hypothesis of isostasy. Concerning 

 some of the fundamental premises, I long ago 

 entertained serious doubts. It has since been 

 fully shown that there was decided error in 

 determining the degree of completeness of the 

 compensation that invalidated the conclusions. 



With the challenge of the basin-range hy- 

 pothesis there has come a demand for citations 

 of concrete examples in support of the theory. 

 Thus far, after the elapse of a full decade and 

 after frequent repetition of the demand, the 

 evidence has not been forthcoming. The 

 Cricket Range, in Utah, recently described in 

 this journal as furnishing a key to the prob- 

 lem, emphasizes this shortcoming. It is not 

 the mere display of profound faulting that is 

 the main desideratum. Abundant evidence of 

 this kind is readily found in nearly every one 

 of the desert ranges. In the majority of cases 

 such faultings are found to have no relations 

 to the present orogeny. Where, according to 

 the hypothesis, the bordering faults should be 

 they are not ; but when found they are usually 



miles out on the interment plains. The pres- 

 ent sharp meeting of mountain and plain is 

 now explained by causes other than disloca- 

 tion, through ordinary stream-corrasion ac- 

 cording to Paige, or through sheet-flood ero- 

 sion as urged by McGee. Under a title of 

 " Locus of Maximum Lateral Deflation in 

 Desert Ranges " I have called attention to its 

 eolative origin. 



By displacement are explained the steep 

 truncations of the transverse ridges of many 

 desert ranges. That these bevelments are 

 really fault-planes bounding the mountain- 

 blocks will have to be more strongly supported 

 than it is now, by direct and unquestionable 

 evidence, before the assertion can be accepted. 

 These rows of truncated ridges seem rather to 

 mark the lines of battle between the plano- 

 rasive advance of eolic degradation from the 

 desert side and the normal stream-corrasion 

 of the more moist mountain areas. 



In the light of the recent advances in our 

 knowledge of the prodigious amount of defla- 

 tion which takes place under climatic condi- 

 tions of aridity it appears that the generally 

 accepted hypothesis of basin-range structure 

 will have to be abandoned and the origin of 

 the desert mountains ascribed to eolic erosion 

 mainly, rather than to local tectonic displace- 

 ment. 



Charles R. Keyes 



note regarding the relation of age to 

 fecundity ' 



In his valuable book on " The Physiology 

 of Reproduction " Marshall,' in a section on 

 the relation of age to fecundity, says (p. 590) : 



The fecundity of the average individual woman 

 may be described, therefore, aa forming a wave, 

 which, starting from sterility, rises somewhat rap- 

 idly to its highest point, and then gradually falls 

 again to sterility. There can be no doubt that 

 animals as a general rule tend to follow a similar 

 law. 



' Papers from the Biological Laboratory of the 

 Maine Agricultural Experiment Station, No. 43. 



= Marshall, F. H. A., "The Physiology of Eepio- 

 duction, " London (Longmans, Green & Co.), 1910, 

 pp. xvii -f- 706. 



