216
THE GEOLOGY OF SOUTH ESSEX.
at Streatham has been considerably deepened, and in addition to the
623 ft. of Chalk mentioned in the Essex Naturalist (Vol. II, p.
148) the borers have met with the following formations :—
The boring was continued to the depth of 1,271 ft., but the last 13 ft.
of core was not brought up. The full details are given (Vol. II, p.
224), and occupy five pages of small type; they are printed for the
first time in this memoir. Thus, the last of the deep borings of
south-eastern England that has penetrated rocks older than any
appearing on the surface, has ended in beds that may be of Triassic,
Carboniferous, or Devonian age.
Chapters 4, 5, and 6, which treat of the Cretaceous Beds, are
largely the work of Mr. Jukes-Browne, who has been making a
special study of the formations of that age, which, as regards the
area included in this memoir, are represented in Essex only by the
few square miles of Chalk about Purfleet and Grays. Chapter 7 is
devoted to the Lower London Tertiaries, or beds between the top of
the Chalk and the base of the London Clay, the various views as to
their affinities and classification being discussed with Mr. Whitaker's
characteristic fairness and ability. The details of the Thanet Sand,
Woolwich and Reading, and Blackheath Beds, occupy chapters 8 to
14. But, as in the half of Essex included in this memoir these
Lower London Tertiaries are seen only in the district between Pur-
fleet and East Tilbury, the Essex sections occupy but an extremely
small proportion of the space. The geological importance of Essex
begins with the overlying London Clay, which is described in chapters
15 and 16, and which is probably nowhere thicker than in the
eastern parts of that county. At Southend, for example, it was
found to be about 440 ft. thick, though, as it was covered not by
Bagshot Beds but by "sand and shingle (beach)" the full thickness
was not ascertained. Though the formation known as London Clay
is almost entirely clay, thin sandy beds occasionally occur in it,
which yield a supply of water when tapped. We learn, from a note