28 LONDON DISTRICT. 



selenite results (see also p. 79). Good crystals of this mineral 

 are not uncommon, but it is from deeper excavations that most 

 of the fossils have been, and are to be, found in and around 

 London. 



When the Basement Bed is exposed fossils may usually be 

 found. Round Bushey and Watford this band consists of brown 

 sandy clay with many fossils in places, also flint pebbles and 

 occasional masses of iron sandstone with Ostrea bellovacensis at 

 the base, and may be as much as 12 ft. in thickness. 



In an excavation for No. 9 Gas-Holder at Beckton, south- 

 east of East Ham, in 1890, at depths of 21 to 30 ft. beneath 

 alluvial deposits (see page 75), the following beds were proved : 



Ft. ins. 



Hard 'grey and brown calcareous sandstone, 

 crowded in places with shells, elsewhere corn- 



London 

 Clay, 



paratively barren, and with occasional flint- 

 pebbles (occurred only in southern part owing 



Basement ^ to depth of gravel in the north) about 1 



Bed. 



Silt (fine grey sand), with decayed shells (1 ft. 



thick in northern part) - - 10 



Clay and shells (poorly preserved) - 39 



Rock band and shells (poorly preserved) - 2 



Woolwich f aay and she]]s _ -56 



T? ai jj- J Silt (boring continued in northern part) 3 9 



ea ing jjard sand (boring continued in northern part) 6 



jt5ecls. I 



The shells from the lower part were not sufficiently preserved for 

 determination, but the hard sandstone at the top yielded about a dozen 

 recognizable species. 



The area of the London Clay is pleasantly undulating and 

 picturesque, with hedgerows well timbered with oak, elm and 

 ash. 



On rising ground, with good natural drainage, the London 

 Clay sites for building are often preferable to those on shallow 

 gravel on lower grounds, where the porous strata are liable to 

 be water-logged, but steep slopes should be avoided as the clay 

 is very apt to slip, causing cracks in the buildings. 



The greater part of the Tube Railways has been cut in the 

 London Clay, which forms an ideal stratum for the purpose. 



Owing to the impermeability of the clay, streams draining it 

 are liable to sudden flooding in wet weather. In London this 

 danger is provided for by relief -sewers, but as late as the 

 beginning of the 19th century the lower course of the West-bourne 

 was so badly flooded that communication between Chelsea and 

 Westminster was only possible in boats; and Stow, the 

 chronicler, records that in the early 17th century a youth of 

 eighteen was drowned as a result of trying to jump the Walbrook 

 at Bucklersbury after a storm. 



CLAYGATE BEDS. 



The Claygate Beds consist essentially of alternations of sand 

 and clay forming a passage from the London Clay to the Ba^shot 



