OCTOBEE 22, 1915] 



SCIENCE 



571 



by bomite, whicli has perhaps not received the 

 attention from mineralogists it deserves. Are 

 we certain that the absence of inclusions of 

 such size as to be visible under the microscope 

 (in this case, the metallographic microscope) 

 necessitates the hypothesis of the existence of 

 solid solution at all? In metallographic study 

 inclusions may be seen to vary more or less 

 continuously from microscopically visible sizes 

 down to the limit of microscopic visibility, 

 which lies in the general neighborhood of 

 0.001 mm. in diameter. This lower limit is 

 determined by the wave-length of light, and 

 has no significance as far as chemical mol- 

 ecules are concerned. It can therefore not be 

 expected that the variation in the size of in- 

 clusions ceases at that particular point; in 

 all probability they also -occur of submicro- 

 scopic size. Accordingly, as an alternative 

 hypothesis to that of Professor Eogers the 

 writer would suggest that the variability in 

 the composition of bomite (normally Cu^FeSJ 

 is due to the presence of submicroscopic in- 

 clusions of one or more of the minerals often 

 occurring as visible inclusions in it, namely 

 chalcocite, chalcopyrite and pyrite. 



Edgar T. "Wherry 

 U. S. National Museum, 

 Washington, E'. C. 



wind gaps 



Are physiographers unconsciously predis- 

 posed in favor of an explanation of topographic 

 phenomena which possesses a dramatic ele- 

 ment as against one which, though quite obvi- 

 ous, involves only the operation of causes 

 which are commonplace? 



An examination of the explanation given of 

 the formation of wind gaps by writers of 

 American text-books on physical geography 

 and geology would seem to answer this question 

 in the affirmative. 



All who treat this topic, so far as I have been 

 able to determine, explain wind gaps — all of 

 them — as deserted water gaps — vestigial struc- 

 tures, as it were, inherited from a certain stage 

 in a past cycle of erosion.^ 



1 Salisbury and Atwood, ' ' Interpretation of 

 Topographic Maps," p. 51. Salisbury, Atwood, 



In this explanation all these writers hark 

 back to the original source, the monograph by 

 Bailey "Willis on " The Northern Appalachians " 

 (American Book Company, 1895). It is true 

 that the monograph itself refers to earlier 

 sources — to the work of Davis and Hayes and 

 Campbell — ^but the constancy of reference by 

 these text-book authors to Snickers Gap, cited 

 in the monograph as a type illustration of a 

 wind gap, and the reproduction of the two 

 diagrammatic maps, there first printed as illus- 

 trations accompanying the explanation of 

 same, indicate this monograph of Bailey "Willis 

 as the true source. 



It is not the purpose of this article to detract 

 from the general admirable treatment of 

 mountain structure contained in the above 

 treatise. It is one of the American physio- 

 graphic classics, replete with that wealth of 

 imagery derived from human activity which 

 so characterizes a writer on physiography of 

 the school of Davis. In that monograph 

 streams now " leap " and now " loiter " ; they 

 " ripple over gravel bars " or " linger between 

 alluvial banks " ; they commit " piracy " and 

 " conquer their neighbors." 



It does seem to the writer, however, that a 

 danger to scientific accuracy lurks in this 

 imagery. An explanation that applies in the 

 vast majority of instances is lost sight of be- 

 cause prosaic. 



As a substitute, therefore, for the — behead- 

 ing — diverting — reversing — stream processes, 

 which must concur in the formation of every 

 wind gap, it would seem, in the view of the 

 writers of the above school of physiography, I 

 would suggest the following : 



A wind gap in the vast majority of instances 

 is simply a col in the top of the divide, 

 notched by the retreat of the sources of two 

 and Barrows, ' ' Text-book on Physiography. ' ' Tarr 

 and Martin, "College Physiography," p. 567. 

 Tarr, "New Physiography," p. 104. Hobbs, 

 "Earth Features and Their Meaning," pp. 176, 

 177. Dryer, "Lessons in Physical Geography," 

 p. 160. Emmerson, "Manual of Physical Geog- 

 raphy." Trafton, "Laboratory and Field Exer- 

 cises in Physical Geography," p. 19. Scott, "An 

 Introduction to Geology," p. 448. Chamberlin 

 and Salisbury, "Geology." Part I., p. 139. 



