too The Irish NahoahsL April, 



and also because the streams flowing in it at present, 

 especiall}^ that on the Dublin side, must have since consider- 

 ably deepened it except at the ver}- summit, where it is 

 comparativel}' dr}'. 



The explanations which might be advanced to account for 

 the formation of the pass are as follows: — 



(i.) That the stream flowed alternately first one way and 

 then the other, the reversals of the drainage being frequent, 

 and caused by some small lateral torrent carrying detritus 

 into the valley during a flood, and so blocking it up tem- 

 porarily and causing the water to flow down the other way. 



To this theor}' there are two fatal objections. In the first 

 place it offers no definite explanation of the waterfall men- 

 tioned above. In the second place the lateral valley, even 

 below the waterfall, is not half the size of Glendoo, and it 

 ought on this supposition to be more nearly twice the size, 

 considering that the erosion has been going on continually 

 in it and only intermittently in each half of the main valley. 



(2.) That the large stream originally flowed into Glencullen, 

 and that the smaller, but more steeply graded stream, flowing 

 from the summit of the original pass towards Dublin, cut 

 backwards until it captured the waters of the larger lateral 

 stream which now, as a consequence, flows north. 



With regard to this explanation it is hard to see how, at its 

 very head where the flow of water must have been so 

 insignificant, it would be possible for the little stream to cut 

 its valley as deep as that of the adjoining large stream. If, 

 however, this once took place, the large stream might easil}', 

 by cutting laterally, break down the final barrier. Also, if at 

 the point of breaking through, the valley of the small stream 

 was 20 or 30 feet lower than that of the large one, the water- 

 lall would also be explained. 



It might be urged that, if the head of the smaller valley was 

 more exposed 10 the beating of wind and rain than the other, 

 the deepening might be efl'ected. The shape of the valley, 

 however, is not such as to suggest erosion of this nature. It 

 is long and narrow and of nearly uniform width and slope of 

 sides, whereas the wind and rain would tend to cause a wide 

 cirque at the head, and to open out the valley lower down and 

 make its slopes gentler. 



