GEOLOGICAL SOCIETY OE DTTBLIX. 
7 
Ho means that could have been adopted to ascertain the age of these 
plants have been neglected; and besides the attention paid to their 
examination by Professor Haughton, I have consulted M. Adolphe 
Brongniart, as already mentioned, whose opinion maybe seen in a trans¬ 
lation of a letter which I lately communicated at one of the Scientific 
Meetings of the Boyal Dublin Society. I may observe, that as I was 
not looking for plants with a view of including the Old Bed Sandstone 
within my line of boundary, I did not originally discover them so low 
down as my friend Mr. Jukes has since done; besides, that colour being 
the order of the day, I limited my researches mainly to the yellow beds, 
discontinuing my search upon reaching the underlying red beds. But 
I shall be ever ready to hear with pleasure of their discovery to the very 
bottom of these rocks, and to recognise them, with Mr. Jukes’ and Mr. 
Haughton’s concurrence, on my Geological Map, as a group of the Car¬ 
boniferous system. I may here observe, that I do not wish to be un¬ 
derstood as aiming at a subversion of the Devonian system, whether 
occurring in Devonshire or elsewhere, my present observations being 
# strictly limited to the Old Bed Sandstone of the south of Ireland. 
The thickness of this series of strata in the south of Ireland may be 
estimated from a consideration of a few typical sections. Thus, at Bally- 
voil Head, looking towards Dungarvan, the strata dip at a steep angle 
nearly vertical, towards the south-west, and we have the whole thick¬ 
ness exhibited from the Silurian to the Yellow Sandstone, which is 
about 2500 feet. Again, the Glasha Biver, near the boundary of the 
county of Waterford, affords a very characteristic section exhibiting 
the thickness of these rocks. The strata in this locality dip nearly 
due north, at an average angle of say 50°, giving a thickness of about 
3200 feet. We have also, at Coolnamuck Bridge, south of the river 
Suir, in the same county, where the rock dips north at an angle of 
about 80°, a section which exhibits a thickness of gray and red Sand¬ 
stone and Conglomerate with red shale of about 2500 feet. In some 
places the thickness may attain a maximum of about 5000 feet. But 
this seems excessive as an average. The thickness of the Yellow Sand¬ 
stone will be about 800 feet, so that we may assume the average 
thickness of the two series to be about 3000 feet. In regard to this 
estimation of thickness, my friend Professor Jukes has kindly lent me 
his assistance in corroborating these statements. It will be understood 
that these thicknesses only apply in the case of a perfect development 
of this series of rocks, as in many cases they will be found to attain 
nothing like such an estimation. 
I may now exhibit a section* which I have lately prepared with 
great care, the horizontal scale being 6 inches to a mile, and the verti¬ 
cal 880 feet to an inch, in which the whole structure of the south of 
Ireland may be seen. In this the granite of the Blackstairs Mountain, 
and the Hill of Brandon, respectively 2400 and 1600 feet above the 
* See Plate V. 
