Correspondence. 379 



ATMOSPHERIC FOECES. 

 To the Editor of the Gteological Magazine. 



Sir, — I am grateful to Professor Jukes for liis handsome acknow- 

 ledgment of the influence exercised on his " views of the amount of 

 atmospheric forces," by my argument of thirty years back, drawn 

 from the volcanic region of Central France. I may mention that 

 another distinguished geologist, Mr. A. Geikie, has also, in his paper 

 on Auvergne, printed in " Notes of Travel by Vacation Tourists," 

 1861,' borne similar testimony to the " enlarged views" he obtaine'd 

 there of the " enormous potency of rain and rivers in effecting the 

 degradation of the land." 



With regard to any difference still existing between Professor 

 Jukes and myself as to the amount of influence exercised by " inter- 

 nal" or subterranean force upon the external configuration of the 

 earth, I think with him that it can amount to little more than a 

 question as to the meaning of the words he employs, since I now 

 understand him to explain that by the phrase " form of the ground" 

 he only means the latest touches given to the surface, such for 

 example as are seen in " the abrasion of a volcanic cone by winds or 

 rain in the course of a few years" (p. 332), not the grander super- 

 ficial inequalities of mountain and valley, dry land and subaqueous 

 hollow, which alone I intended to refer in a large degree to the 

 agency of subterranean forces. 



Some little misunderstanding may yet remain under cover of the 

 Professor's use of the word " direct," as wlien he says (p. 332) 

 '' The direct effect of earthquakes in cracking, or bending the surface 

 are surely very uisignificant, etc." Now, if bj'' "earthquake" is 

 meant (as of course must be meant) those oscillations of level which 

 have in some localities carried up beds of recent sea-shells to heights 

 of hundreds of feet, and Tertiary marine strata to that of thousands, 

 above the sea level, while portions of the same beds, once continuous 

 with these, and even now at no great horizontal distance, have re- 

 mained unmoved, or have been proportionately depressed, I cannot 

 understand how such effects can be styled " insignificant," or be 

 considered of little moment in an inquiry as to the causes of the 

 " form of the ground." 



The paramount influence exercised by subterranean energy in 

 determining the configuration of the earth's surface, might indeed be 

 inferred a priori from the considerations, (1) That no upheaval or 

 depression of surface-rocks could take place without leaving propor- 

 tionate inequalities of superficial level; and (2) That the inequalities 

 so produced must have always largely determined both the direction 

 and the force of the external denuding agencies. That such changes 

 of level have been taking place throughout all time down to the 

 present day, and upon the largest scale, though probably never on 

 the large scale "per saltum," but rather by slow continuous move- 

 ments, or frequently repeated jerks, such as are characteristic of 



