432 Correspondence — Mr. A. J. Jukes Browne. 



depends upon the truth of this supposition. Jukes never supposed 

 the limestone to have had this extension at the period when the 

 erosion of the valleys was commenced ; on the contrary, his theory is 

 expressly based on the supposition that the surface over which 

 the rivers originally ran was a plain of marine denudation, cutting 

 across the folds and contortions of the rocks, 



Mr. Kinahan now affirms that the Carboniferous Limestone thins 

 out southward, and that its original thickness in the valleys of the 

 Lee, Bride and Blackwater was not so great as Jukes had supposed ; 

 but, assuming this to be true, the argument which Mr. Kinahan 

 founds upon this premiss is equally defective in logic and in gram- 

 mar ; — he thinks that " as the theory was founded in a county and on 

 suppositions which were afterwards found to be erroneous," he is 

 justified in saying that it falls to the ground. I can onty express 

 my astonishment that Mr. Kinahan should consider any part of his 

 own country to be erroneous, — the imputation is so utterly incon- 

 sistent with his patriotism that in a Dogberry sense it seems to be 

 " flat burglary." As regards the erroneous supposition (for there is 

 only one), my answer is simply that Jukes's theory was not founded 

 upon it ; Mr. Kinahan himself admits this in the passage I quoted 

 from his earlier work, distinctly saying that it did not much affect 

 the subject, " as some of the other rocks are nearly as easily denuded 

 as limestone." 



It will be obvious to others, however, that, if the thickness of the 

 limestone was never sufficient to fill up the troughs to the level of 

 the original plain, their centre would be occupied by a strip of Coal- 

 measures ; and that Jukes's reasoning would still remain the same, 

 for his theory does not depend on the universal presence of lime- 

 stone, but on the fact of the rocks in the synclinals being more 

 easily denuded than those of the anticlinals. 



Having entered the lists in defence of my late uncle's views on 

 this subject, I am glad of the opportunity of noticing a difficulty 

 raised by Prof. Hull in .his " Physical Geology of Ireland." Prof. 

 Hull accepts the theory in general, but dissents from its application 

 to the Blackwater, because the point where the present river is 

 deflected southward does not coincide with the influx of any stream 

 from the north. But Jukes's main object was to explain the forma- 

 tion of the transverse ravines, and he looked back to a time when the 

 longitudinal part of the Blackwater Valley did not exist, and when 

 the two brooks from the north " may have united their waters some- 

 where about the northern end of the Dromana Bavine." I am 

 quoting Jukes's own words, and feel sure that were he now alive he 

 would make a similar answer, and would add that the present actual 

 point of deflection has been fixed by the subsequent changes in the 

 river-course since the establishment of the Dromana Bavine. I trust 

 that Prof. Hull will reconsider this point before bringing out a 

 second edition of his work, and may see his way to accept the theory 

 without excepting the Valley of the Blackwater. 



Geol. Surv. of England, A. J. JuKES BROWNE. 



Alford, Aug. 8, 1879. 



