A HISTORY OF KENT Secondary division and embrace portions of the Paleozoic or Primary rocks also. By reference to Table I. on the next page the extent of these additions to our knowledge will be readily grasped. The geology of the county therefore falls naturally under two heads : {a) the study of the outcropping strata and of the features to which they give rise ; and (^) the study of the deep-seated rocks found only in the borings and having little or no effect upon the outline of the present surface. It is customary in geological literature to commence with the description of the oldest formation and to work upward through the sequence. But for our present purpose this method is hardly suitable, since the most difficult and technical part of our subject, and that respect- ing which our knowledge is necessarily restricted, would have to be considered first. The simpler method will be to give precedence to the description of the surface formations of the county in the above- mentioned order, and to discuss the results of the deep borings after- wards under a separate heading. The tables which follow will serve as indices to the stratigraphical sequence : the first gives a summary of the rock systems commonly adopted as the geological time-scale, with an indication of the portions known to exist in Kent ; and the second contains the fuller classification and sub-division of the Kentish strata. SUMMARY OF ROCK-SYSTEMS Classification Rock-systems Relation to Kent Cainozoic or Tertiary Recent Pleistocene Pliocene Miocene Oligocene Eocene Fairly represented Partly represented Scantily represented Absent Absent Widely represented Mesozoic or Secondary Upper Cretaceous .... Lower Cretaceous .... Upper Jurassic Middle Jurassic Lower Jurassic Triassic Widely represented Widely represented Fully represented in deep borings Fully represented in deep borings Partly represented in deep borings Scantily represented in deep borings Paljeozoic or Primary Permian Carboniferous Devonian Silurian Ordovician Cambrian Not known to exist Upper part proved in two deep borings Possibly represented in deep borings 1 Some part certain to exist deep under- V ground, and possibly reached in one J boring Unknown Eozoic Azoic ? Pre-Cambrian Archxan Unknown Unknown
