} 
—-, a, 
TRANSACTIONS OF THE SECTIONS. 73 
plank with a bolt was discovered, showing that this part of the bank had no great 
antiquity. An Artesian well has also been made to supply Block-house Fort, which 
for the first sixty feet exhibits nothing but clear shingle, and then a layer of sandy clay 
full of common oyster-shells, another example of the great changes in the ancient 
coast and sea-bottom. 
Hydrography of the British Isles. By Avcustus Petermany, F.R.G.S,. 
Mr. A, Petermann exhibited a new Hydrographic Map of the British Isles, on 
which about 1550 rivers are distinguished by names, 480 lakes and ponds, and 40 
waterfalls; the canals with their altitude, as well as that of the rivers and lakes, and 
the great drains in the fen districts. It was stated that there are 20 rivers in En- 
oe 10 in Scotland, and 10 in Ireland, each draining 500 square miles and up- 
wards, 
Of these, 18 drain an area = 500 to 1000 sq. miles. 
aig .. 1000 ,, | 2000 
ee) ree signs Grong... 
. These last eight are,— 
The Humber (including Trent and Ouse), to Spurn Point,,. 9550 sq, miles, 
Severn, to Flatholm Light ...........cssesescseeees tesseeee 8580 
Shannon, to Loop Head and Kerry Head....,.......... 6946 
Thames (including Medway), to Nore Light...,........ 6160 
Barrow...00e seeeeeseeees fcohosting: Aoprrocasencne Sasa chess 3410 
RreHAOISE sy acstresicnng se snag dusessycpeapnaats open sen’ seveee 2960 
HI ica aegantesssicscs espaarah sev usecenscsassaceecesmetsncens 2345 
Tay, as far as Rhynd ...........00- Seaptinannae pnaésh anaes 2250 
The River Amazons drains a tract of 2,275,000 square miles, 
On some points connected with the Physical Geology of the Silurian district 
between Builth and Pen-y-bont, Radnorshire. By Prof. Ramsay, F.G.S. 
In this paper Professor Ramsay first laid down certain established geological pro- 
positions, on which much of the reasoning in the communication depended. 
When a stratuin rests unconformably on the upturned edges of another series of 
_ strata, the lower rocks were denuded, either previous to or during the deposition of 
the higher stratum, and we know of no power at any considerable depth beneath the 
level of the sea fitted to effect such phenomena, which therefore took place either above 
or at its surface. 
In the district described (near Builth in Breconshire), the Wenlock shale rests 
unconformably on the Llandeilo flags, which there consist of black slates associated 
with beds of trap and volcanic ashes. These rocks having been disturbed and raised 
above the level of the sea, formed the land round which the lowest beds of the un- 
conformable Wenlock shale were deposited, and gradually sinking beneath the level of 
_ the sea was covered up by higher Silurian strata, which accumulated above it to the 
vertical thickness of 5000 feet. A part at least of the old red sandstone was added 
_ to this, and during subsequent oscillations of level these higher rocks (beneath which 
_ the old land had been so long and deeply buried) were removed, and the Llandeilo 
_ flags of the district are now land for the second time. 
The sections exhibited also afforded data, by which could be ascertained the angles - 
of inclination of the Llandeilo flags at the time they lay under the Wenlock and other 
superincumbent beds, previous to the disturbance that raised these latter formations 
into an anticlinal curve, the lowest bed of which rested unconformably on the up- 
turned and denuded edges of the Llandeilo flags. 
They also indicated a method by which it may sometimes he possible to determine 
the vertical thickness of accumulations above certain other deposits, thus pointing to 
a means of forming a proximate idea of the degree of heat the latter may once have 
endured, supposing the same ratio of increase of temperature as we descend beneath 
the surface to have existed at that geological epoch that now obtains. 
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