Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/297

 {| style="font-size:85%;line-height:140%;"
 * No. || || Feet.
 * align="right" valign="top" | 12. || Striped sand, yellow, fine, and iron-shot || align="right" valign="bottom" | 10
 * align="right" valign="top" |13. || Alluvium || align="right" | ─
 * align="right" valign="top" | || || align="right" | ──
 * align="right" valign="top" colspan="2" | total || align="right" | 91
 * }
 * align="right" valign="top" | || || align="right" | ──
 * align="right" valign="top" colspan="2" | total || align="right" | 91
 * }
 * align="right" valign="top" colspan="2" | total || align="right" | 91
 * }

At a point still higher on this hill than No. 12, is a thick bed of dark blue clay, without shells, which is used to make tiles and bricks, and which appears to continue upwards trom this brick kiln to the summit of the hill, forming a thick cap over the sands and clays mentioned in the section, and is probably an outlying hummock of the London clay, separated only by a small valley from the extensive mass of that stratum which is found two miles south-west in the Sydenham hills, and being placed between and connecting them with the London clay of Shooter's hill. Mr.Webster mentions (Geol. Trans. v. 2, p. 235) that rounded flints are found in the sand strata, at the bottom of the blue or London clay, in several parts of the London basin. And again (p. 185), that the abundant supply of water which is constantly found in boring through the same clay, indicates an extensive deposition of three beds of sand. The sandy strata containing pebbles, and the watery sand thus alluded to appear to be the continuation of the upper strata of the plastic clay formation, and connected with those of Loam Pit Hill.

In the shaft at the northern extremity of the tunnel under the Thames, near Rotherhithe, these same beds were found, covered by more than 30 feet of London clay, although from their rapid rise under the bed of the Thames towards the south, the shaft on the south side (of which Mr. Webster has given a section, p. 197) exhibits only nine feet of this clay incumbent on the watery gravel and subjacent beds of the plastic clay formation.

A curious section is preserved in Sir C. Wren's Parentalla (p. 285), obtained in preparing the foundations of the present cathedral church of St. Paul, In London.

The Surveyor observed that the foundations of the old church stood upon a layer of very clan hard pot earth, which he therefore judged firm enough to support the new building; and on digging wells in several places he found this pot earth to be about six feet thick and more, on the north side of the church yard, but thinner and thinner towards the south, till it was scarce four feet upon the declivity of the hull. Below this he found noting but dry sand, mixed sometimes unequally, but loose, so that it would run through the fingers. He went on till he came to water and sand mixed with perriwincles and other sea shells; these were about the level of low water mark. He continued boring till he came to natural hard clay.

The upper stratum of pot earth had been used at a Roman pottery, near the N.E. angle of the present church, where they found urns, sacrificing vessels, and other pottery in great abundance, and were interrupted in digging the foundation of the N.E. angle of the church, by the quarry from which the pot earth had been extracted: the subjacent sand and gravel beds being considered too loose to support the weight of the intended building, it was thought necessary to secure this part of the foundation by erecting it upon an arch, The outer or N.E. pier of this arch stands in the old clay pit, in a shaft sunk to receive it more than 40 feet below the stratum of pot earth that had been removed, and descending through the beds of sand and gravel above mentioned, to the subjacent stratum of hard clay.