320 



SCIENCE. 



[Vol. XII. No. 307 



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 ztion will be furnished 

 i with the character of 



Synclinal Mountains and Anticlinal Valleys. 



In the recent reading of two very different books a statement is 

 encountered that seems incomplete, and to a certain extent mislead- 

 ing, regarding the origin of synclinal mountains and their accom- 

 panying anticlinal valleys. The first book is an advanced mono- 

 graph, entitled ' Les Formes du Terrain,' by La Noe and Margerie, 

 in which it is stated (p. 150) that " finally a very remarkable conse- 

 quence of erosion in regions of folded structure is to give rise to an 

 ultimate arrangement of relief in which the original depressions are 

 replaced by elevations, and OT'(;(?7/(?rj«." This generalization seems 

 too broad. In the first place, the only ultimate form of land-sculp- 

 ture is the base-level plain, down to which every surface must be 

 reduced, whatever its structure, if time be allowed. Of this the 

 authors are undoubtedly aware, and their term ' finale ' should per- 

 haps be rendered ' late ' rather than ' ultimate.' But, in the second 

 place, it is by no means essential that late forms developed by ero- 

 sion on folded structures should present the inversion referred to. 

 It is not a necessary or even a general result of progressive denu- 

 dation, but simply a special result of a certain relative position of 

 hard beds and the controlling base-level. For example : let the 

 dotted stratum in the accompanying figure be a hard sandstone, 

 while the other beds are soft shales and limestones. If the base- 

 level be at A, a little below the arches of the hard bed but above its 

 troughs, a late form assumed in the progress of denuation will be 

 broad synclinal lowlands between anticlinal ridges (shown in long- 

 broken lines, ). In this case the late form corre- 

 sponds to the original structural surface. But if the base-level is at 

 C, the opportunity for quick erosion that is afforded on the lower 

 soft beds, when they are discovered by the breaching of the anti- 

 clinal crests, will soon cause the chief water-courses to abandon 

 the synclinal axes that they had before followed, and excavate their 

 valleys along the anticlinal axes. In this case the late form (shown 



by short-broken lines, ) consists of synclinal ridges or 



mountains, and there has been an inversion from the original struc- 

 tural surface. It may be added that this result is favored if the 



region is first base-levelled at an altitude like B (the ultimate form 

 in this cycle of development being shown in the dotted line ....), 

 and then bodily elevated so that the base-level falls to C ; and this, 

 I think (following suggestions from Gilbert and McGee), has been 

 the case with the Appalachians. 



The second book referred to is Hinman's ' Eclectic Physical 

 Geography,' recently published. The Jura Mountains are taken as 

 examples of young forms, in which the anticlinals are ridges : the 

 Appalachians are chosen as examples of old forms, in which many 

 anticlinals are worn down to valleys. By the time the Jura " have 

 suffered erosion as long as the Appalachians, the present position of 

 the mountains and valleys will have been reversed" (p. 261). It 



■-v" 



seems pretty certain that when the Jura have suffered erosion as long 

 as the Appalachians, they will be worn down flat ; and whether an 

 inversion of ridges and valleys will take place, or not, during this 

 erosion, is entirely a special result of the relative attitude of hard 

 beds and base-level, as above illustrated, and by no means open to 

 unqualified prediction on general principles. 



From finding the above statements concerning the origin of syn- 

 clinal mountains in two books whose objects are so dissimilar, I 

 have inferred that they represent a general belief, in which a special 

 case is conceived to be a general one ; hence this brief note on the 

 subject. The question is a small one, and unnecessary in general 

 accounts of geographic forms ; but when the progress of denudation 

 is alluded to, and early and late forms are distinguished, it is essen- 

 tial that the principles of classification should be clearly stated, it 

 one would gain a full understanding of the systematic develop- 

 ment of the surface of the earth. W. M. Davis. 



Philadelptiia, Penn., Dec. 12. 



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