688 ABSTRACTS OF PAPERS 



mean temperature of the earth's surface. Increase of C0 2 probably must pro- 

 duce rise of temperature and increased humidity; decrease of C0 2 probably 

 must produce contrary effects. These two agencies cooperate, since elevation 

 of land must produce decrease of C0 2 . Though we can not attribute to changes 

 in the eccentricity of the earth's orbit the dominant influence claimed for it by 

 Cross, it seems probable that great changes of eccentricity would produce some 

 climatic effect. Climatic changes must be supposed to result from a complex 

 of causes, some of which are known and others probably unknown. 



Prof. C. W. Brown: The question occurs to me that these curves — which, 

 allow me to say, must represent an enormous amount of time and labor — that 

 show approximate agreement at times between solar and terrestrial dynamic 

 phenomena and terrestrial climate may have been partially obscured by mixing 

 the data concerning seismic and volcanic phenomena of large land-masses, .rela- 

 tively stable, with those of smaller and less stable regions more susceptible to 

 this unknown cosmic stimulus. 



Smaller impulses would affect regions of the crust in a state of delicate 

 equilibrium at once, yet they might affect more stable land-masses until the 

 accumulated stresses resulted in rupture after a lag period of three, six, or 

 many months later. A relatively feeble impulse of the waning portion of the 

 period of activity might, therefore, serve as a trigger to set off the accumula- 

 tion of strains in the more resistant segments. The maxima of the unstable 

 areas might then be moved along or partially neutralized by the maxima of the 

 larger, less easily affected areas occurring on- the down slopes or at the minima 

 of the former. 



The difficulties resulting from lack of coincidence of the curves might possibly 

 be avoided if the curves compiled from regions in unstable equilibrium were 

 compared with each other, as well as those from more stable areas were studied 

 separately. I feel diffident about bringing up this point and suggesting more 

 labor, when it may have no value at all. 



Dr. A. L. Day : In general any pair of periodic curves may be combined in 

 this way and will produce a series of maxima and minima in regular succession 

 without proving anything more than their individual periodicity. In the pair 

 of curves before us, from which strictly characteristic features have been re- 

 moved by averaging, some independent proof other than the mere alternation of 

 maxima and minima is, therefore, necessary to establish any relation between 

 them. 



PIEDMONT TERRACES OF TEE NORTHERN APPALACHIANS AND THEIR 



MODE OF ORIGIN 



BY JOSEPH BARBELL 



(Abstract) 



Physiographers are accustomed to build on the results of structural and 

 historical studies in order to explain the "origin of the present surface forms. 

 Here the opposite method is followed — one which it is thought might be more 

 widely adopted by historical geologists — building on a knowledge of the pres- 

 ent relations of surface forms to rock structures in order to explain the past. 

 In this first paper is given a description of the significant forms and the argu- 

 ments as to mode of origin. In the second paper this data is combined with 



