GEOLOGY OF MY GARDEN. 21 



Beyond this bed of sand, on the south, the chalk comes to the surface, 

 and dips under my garden, not coming near the surface again till South 

 Mimms and Hatfield, north of London. On the north of my garden, 

 blue clay and the whole series of Lower Tertiary beds of clay and sand 

 exist (plate 5). 



Geologically speaking, we are at the very edge of the London Basin ; 

 a section of which, copied from the Geological Survey by the kindness 

 of Sir Henry James, is given in plate 4. Under London, as the centre 

 of the basin, we have a depth of about one hundred feet of these 

 Lower Tertiary beds, of which my bed of sand is the last : above these 

 we have another one hundred feet of blue clay. This is covered with 

 a bed of gravel, and finally with a layer of about sixteen feet of 

 waste earth, and occasionally of peat. These depths vary somewhat in 

 different localities ; but, from an examination of the sections of many 

 London wells, these measurements may be taken as generally accurate. 



The dip of the strata through my garden has been kindly drawn for 

 me by Mr. Alfred Tyler the geologist, and his section (plate 4) shows 

 how rapidly the different strata fall towards the Culvers, the property 

 of Mr. Gassiot, as they dip through the grounds of Shepley House, the 

 residence of Mr. Tyler. 



To the south of us the chalk comes to the surface and rises to above 

 850 feet. The chalk consists of two layers, the upper chalk with flints, 

 and the lower chalk without flints. Flints occur in horizontal layers, 

 which may be traced for miles ; these are crossed by veins more or less 

 perpendicular, like mineral veins in Cornwall. These vary from a hair's 

 breadth to a foot in width. These cracks, like the layers of flints, run 

 for miles, and sometimes there is a dislocation of the flints where the 

 cracks occur, one side being higher than the other, as may be well 

 seen at the Sutton chalk-pit. These cracks are the underground rivers 

 or watercourses of the chalk ; and whenever in sinking a well one of 

 these cracks is tapped, a valuable flow of water is secured. When, on 

 the contrary, only the solid chalk is pierced, a very small supply of 

 water is obtained. The existence of these fissures is of great value 

 to mankind. 



