By Pror. Epwarp HUuLL1, M.A., F.R.S. | 15 
why has the river itself disappeared? The answer to this question 
is not difficult ; nay, is almost obvious. During the progress of 
denudation some districts would be more rapidly stript of their 
rocks and strata than others, The loose beds of shale, flagstone, 
sandstone, coal, &ec., of the Carboniferous area would be disinte- 
grated and carried away by the rains and rivers much more 
rapidly than the solid granite, tough schists, and quartzites ; 
all highly tilted, and therefore, more suited by position to resist 
weathering than the nearly horizontal beds of the Carboniferous 
system. ‘Thus, then, while all rocks exposed to the atmosphere 
were subjected to waste, those of the Carboniferous area were 
carried away more rapidly than those of the country to the south- 
wards. In other words, the Carboniferous plain became relatively. 
lowered more rapidly than the granitic area to the south, and 
ultimately so much so, that the granite and Silurian ridge became 
the watershed; and the stream, which had up to this time flowed 
through “TheScalp,” was diverted from its course, and began to run 
down towards the position of the existing valley of Shanganagh 
River.* 
Pug. 2.—Geological section through the Central Plain, and Wicklow 
Mountains, showing existing Stratification. 
THE SUGAR LOAF 
THE RIVER 
SCALP DARGLE 
; t 
RIVER RIVER 
~ DODDER SHANGANAGH 
SECTIONEA. i i 




LIMESTONE SS 
Se ee ‘3 SR 
os <a pe, 
Gee RGEAR: NSE IleeT ecb ks SCHIST. AND QUARTZITE 
It is, in fact, exceedingly probable that the stream now called 
“The Shanganagh” River originally flowed through the Scalp, 
and that its present course and direction is due to the change in 
the relative levels of the granite hills and the limestone plain. 
The vicissitudes of the granitic mountains are themselves remark- 
able. Originally upreared and developed, probably at the same 
time as the mountains of Donegal, Mayo, and Galway, viz., at 
the close of the Lower Silurian period, they were again nearly 
buried (as we may say) under Carboniferous strata, but were 
* Suppose, for example, that the granitic and schistose area was denuded at the rate of 
one-tenth of an inch in 1,000 years, and that of the Carboniferous at the rate of one inch 
in the same period, it isevident that the Carboniferous and granitic areas would ultimately 
assume the relative positions they now occupy. 
