492 W. J. SOLLAS ON THE SILURIAN DISTRICT 
clinal axis and on the south side of the synclinal axis, in all 2684 
yards, we have left a distance of 2024 yards, which is covered by 
the remaining Old Red beds and by the top of the Silurian series. 
This horizontal distance corresponds to a vertical thickness of 
3036 feet for beds dipping at an average inclination of 30°. We 
finally deduct 545 feet, the thickness of the Silurian beds which are 
known to lie above the Rhymney Grit, and then have a remainder 
of 2491 feet, which, added to the 1782 feet previously determined, 
gives us 4273 feet as the total thickness of the Old Red Sandstone 
in this part of South Wales. Of course this determination is only 
approximate and depends partly on an assumption; but I think, if 
found to err at all, it will rather be on the side of over- than under- 
estimation of the true thickness. Thus, in passing from the Vans of 
Brecon, on the north side of the coal-basin, to our area, a distance of 
about 25 miles, the Old Red Sandstone has lost more than half its 
thickness. Along with this we find a corresponding thinning-out 
of the cornstones of the formation, which were much more sparingly 
represented here than.to the north. This is in full accordance with 
an opinion which L have for a long time entertained, that these 
calcareous beds are mechanical sedimentary deposits, not imme- 
diately derived from organic remains, but only indirectly through 
the denudation of previously formed limestone beds, belonging pro- 
bably to the Cambrian formation. 
It would be foreign to the subject of this paper to enlarge further 
on this topic, and I hope to be able to say something more about it 
on another occasion; but I may perhaps be allowed to add now 
that although calcaréous beds are generally regarded as in all cases 
due to deposition from solution either by evaporation or through the 
immediate agency of living things, yet there are several instances in 
which they have not been so formed, but have on the contrary been 
derived from sediments which have been carried in suspension and 
strewn out in deposits, in just the same fashion as clay or any other 
mechanical sediment may be. The conglomerates of Mountain 
Limestone formed on ancient beaches of the Lias are a case in point ; 
and the pebbles of these conglomerates are frequently as well 
rounded as any we can find on a beach at the present day. What 
has become, then, of the asperities and angles which have been 
worn away during the rounding of the pebbles? The usual reply 
is, ‘¢ Dissolved in the surrounding sea-water;” but it seems to me 
that there is always the alternative possibility that it has been 
carried away as mud in suspension, just like any other mud; and if 
so, the deposition of such calcareous mud would go far towards 
explaining the occurrence of calcareous septaria and other nodules, 
which so frequently occur in red deposits like those of the Old Red 
Sandstone and the Trias, as well as the formation of the muddy 
Lias limestones of Penarth Cliffs, which, with their flat even bedding 
and numerous intercalations of black shales, certainly do not 
suggest an organic origin. A few oysters and such like shells may 
have contributed to their growth; but their general appearance 
certainly is that of strata formed from sediment carried in suspen- 
