II 
especially at Bagshot Heath, but it must formerly have had a 
much greater northern and eastward extension, since it is represented 
by isolated patches not only in the North of London, but far into 
Essex. The minerals in the sands of Hampstead Heath have been 
carefully studied by Mr. Allan Dick. The Bagshot sands, more 
or less iron-shot, repose on the London clay, into which they pass 
insensibly downwards, the upper part of the clay being sandy or 
loamy. At the junction of the sands with the stiff clay, water 
which has soaked through the permeable porous beds will be 
thrown out; and hence the source of many springs and streams. 
The water, in percolating through the ferruginous sands, dissolves 
more or less iron, so that it may form a chalybeate spring, such as 
the well-known source in Well-walk, to which Hampstead at one 
time owed its reputation as a spa, and became the favourite resort 
of fashionable society. The impermeable clay holds up the waters 
of the Hampstead and Highgate ponds. The old Fleet river, 
the Tybourne, and the head waters of the Brent, rise partly from 
the drainage of the clay land, and partly from the junction of the 
pervious sands with the impervious clay. The London clay, which 
covers a very large part of the London Basin, reaches a maximum 
thickness of about 500 feet in South Essex and in the Isle of Sheppey. 
In the old boring on the Lower Heath at Hampstead the clay was 
289 feet thick. At the Orphan School at Haverstock Hill the 
thickness is 223 feet; and further southwards, at a lower level 
it is less, being reduced at the old Hampstead Road Reservoir (now 
Tolmers Square) to only 59 feet. The London clay contains 
' numerous concretions of argillaceous limestone, known as “ turtle 
stones,” ‘‘septaria,’”’ or “‘ cement stones,” the last name having a 
__ reference to their former use in making “‘ Roman cement.” These 
_ septarian nodules frequently enclose fossils as nuclei. There are 
_ also found in the London clay small nodules rich in phosphate of 
_ lime, crystals of selenite, often known as ‘‘ congealed water,” 
_ and masses of iron-pyrites, or disulphide of iron. The fossils of 
_ the London clay, which have occasionally been found abundantly 
at Hampstead, Highgate, and Primrose Hill, and other localities 
in the North of London, suggest a warm, almost tropical climate. 
_ They include cones, cowries, spindle shells, and several species 
_ of nautilus. In the Isle of Sheppey fossil fruits are common in the 
_ clay, and many of these suggest tropical relations, being akin . 
_ to the nipa palms of Bengal and the Spice Islands, the oil palms of. 
_ Africa, and other plants of hot climates. The London clay rests 
-_ na series of beds known as the Lower London Tertiaries, which 
in turn repose upon the chalk. Numerous deep wells in London — 
draw their supplies from the chalk, but the attempt to obtain water 
from Lower Greensand made at the famous Kentish Town boring 
