ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixvii 



skirts of the Old Red Sandstone follows the course of the Towy. In 

 this undulating character the southern part of the general trough 

 differs from the more northern portion, where, the beds between the 

 Merioneth and the Berwyns anticlinal dip generally with great 

 regularity to the S.E., but are broken by enormous longitudinal 

 faults. Consistently with these undulations, it has been shown that 

 beds which appear at the surface on the shores of Cardigan Bay, 

 near Aberystwith, are also found near the south-eastern boundary of 

 the Silurian district*. These beds belong to the Bala series. The 

 beds S.E. of Cader Idris, belonging to the lower fossiliferous beds of 

 Merionethshire, were long since observed by Professor Sedgwick to 

 dip beneath them, but their exact place in the series of deposits had 

 not then been determined. 



The depth of the trough in S. Wales would be nearly uniform 

 between the limits as above defined on the N.E. and S.W., with the 

 exception of the variations arising from the intermediate longitudinal 

 ridges and furrows. Its general depth may not, perhaps, differ much 

 from the deepest part of the most eastern ti'ough of N. Wales im- 

 mediately west of the Berwyns. 



If we now pass south-eastwards across the anticlinal line running 

 from St. Bride's Bay to Builth, we find the Silurian beds descending 

 with a very rapid dip beneath the Old Red Sandstone, the Mountain 

 Limestone, and the Coal-measures of South Wales. The Old Red 

 Sandstone along this boundary dips very persistently to the S.E.f, 

 but deviates more to the south as it approaches the basset edge of 

 the Mountain Limestone, where that edge assumes an east and west 

 direction. Proceeding further to the N.E. along the line already 

 indicated from Builth to the Longmynd, we also observe the dip on 

 the S.E. of that line, though subject to many local variations, 

 generally to the S.E. If then we examine the position of our ideal 

 stratum in the region now under consideration, we shall find it 

 regularly descending under the Old Red Sandstone, but more rapidly 

 as it approaches the Coal district. Supposing the Silurians to be 

 continued with undiminished thickness under the last-mentioned 

 formation, our ideal stratum will descend to a level many thousand 

 feet lower than on the north-western side of the anticlmal along the 

 boundary of the Old Red Sandstone. Professor Ramsay estimates J; 

 the thickness of the Old Red Sandstone to vary from 4000 to 7000 

 feet, and that of the Carboniferous Limestone from 100 to 2000 feet, 

 and that of the Coal-measures from 8000 to 12,000 feet; so that 

 the upper part of the Silurian series, in the lowest part of the Coal- 

 field, is probably at a depth not less than 15,000 feet. To this must 

 be added the whole thickness of the Upper and Lower Silurians in 

 the same locality. At this southern end, therefore, of the Principality, 

 we have an enormous geological depression, similar to that in Den- 



* " Sketch of the Structure of North and South Wales," by Professor Ramsay, 

 Quart. Journ. Geol. Soc. vol. iv. p. 297, and Sections of the Geological Survey. ' 



t Geological Map of the Ordnance Survey ; and the Map of Sir R. I. Murchison's 

 Silurian System. 



t Memoirs of the Geological Survey, vol. i. p. 316. 



e2 



