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EXPLANATION OF MAP.

As the Map accompanying this volume differs in some respocts from other
geological maps of England and Wales, it may be well briefly to indicate the
changes introduced.

The index of colours has been arranged to show the comparative thicknesses
of the formations.

The Cambrian and Silurian rocks are classified according to Sedgwick, so
that the former group includes the Lower Silurian of Murchison.

The Devonian rocks are shown in the index as representing portions of both
Old Red Sandstone and Lower Carboniferous Rocks, which seoms to be their
true position in the geological scale.

The Neocomian (Wealden and Lower Greensand) or Lower Cretaceous
strata are separated from the true or Upper Cretaceous strata.

The Drift deposits (Boulder Clay and Gravels) which it is attempted to
show, comprise a very small portion of those which are scattered over the greater
part of the country; but as yet they have been mapped only in certain
areas where they are perhaps most extensively developed, and these areas are
shown. It must be remembered, however, that while they obscure so much of
the Chalk and Tertiary strata in the Eastern counties, these older deposits are
generally exposed in the valleys, although in tracts too small to display them
upon the Map.

LIST OF WOODCUTS.

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110

13. Section of Triassic Rocks near Denbury Farm, Wiveliscombe

137

14. Section of Lower Lias and White Lias, near Bath

145

15. General section of the Cotteswold Hills

176

16. Purbeck and Portland Beds at Tilly Whim, near Swanage

207

17. General section across the Wealden district

211

18. Section at Snowdown, Chard

245

19. General section of the London Basin

257

20. Section across Headon Hill and High Down, Isle of Wight

274

21. Chalk pit at Whitlingham, near Norwich

287

22. Section at Writtle, near Chelmsford

314

23. Diagram-section of Landslip near Lyme Regis .

353

24. Diagram illustrating the encroachment of the sea near Watchet 398

25. The Cheesewring

411

26. Rippon Tors, Dartmoor

411

27. Railway-cutting, near Uphill

423

28. The Cheddar Cliffs

Frontispiece

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