Reports & Proceedings — Geological Society of London. 187 



imknown, namely the southern counties and a tract extending north- 

 westwards from the estuary of tlie Thames ; but, between the two, 

 contour-lines at 500, 1,000, and 1,500 feet below, sea-level can be 

 drawn for considerable distances. The form of the platform thus 

 illustrated shows no connexion with its geological structure, so far as 

 that is known, and may be attributed to planation, for the most part 

 by marine action, and to warping by the post-Oligocene movements 

 which produced the dominant structure of the South-East of England. 



To illustrate the effect of these movements a second map is con- 

 structed, showing contour-lines in the base of the Gault at intervals 

 of 500 feet, extending from 500 feet above the sea to 2,500 feet below 

 it. The result is to give a comprehensive view of the warping 

 undergone by the Upper Cretaceous and Tertiary rocks and of the 

 magnitude of the movements. The height above sea-level to which 

 the base of the Gault has been raised, and the depth below sea-level 

 to which it has been depressed, can be estimated approximately at 

 every point from the contouring. It thus becomes possible to see 

 what corrections are necessary to restore the base of the Gault to 

 horizontalitj^, and thus to eliminate the effects of the post-Oligocene 

 movements. 



By making this correction in the first map, the Palaeozoic platform is 

 restored to the form that it possessed before those movements came into 

 operation. A third map, thus constructed, shows that considerable 

 changes were effected. A tract of high elevation clearly defines 

 itself under London and near Harwich. On the northern and southern 

 sides the contour-lines run with much regularity, and those on the 

 south give evidence of a bold slope continuing downwards far in that 

 direction. It appears that the London Syncline has come into existence 

 in an area that was long one of elevation, and that the Wealden 

 Anticline has been superimposed upon a region of depression. 



It is further pointed out that, for a continuance of these investi- 

 gations, we are dependent upon the making of borings for coal or 

 water ; but that no machinery has been created for the systematic 

 registration of borings, notwithstanding the recommendation made 

 in the Report of the Hoyal Commission on Coal Supplies. The 

 information gained in a borehole may come to hand by chance, or it 

 may be lost. Frequently no permanent record is kept of the site, 

 diameter, or depth of boreholes. 



It is noted with pleasure that a precise levelling is being carried 

 out by the Ordnance Survey to replace the levelling made in 1841 

 to 1859, which was not of modern precision, and was recorded by 

 marks which were not permanent. It is pointed out that in work of 

 the precision contemplated factors of unknown value may have to be 

 taken into account, such for example as earth-tides or the effect of 

 the oceanic tide upon coastal regions, but more especially the variations 

 in gravity which are known to exist, although they have never been 

 systematically examined in the British Isles. Observations are in 

 progress in India and in many foreign countries ; but in our own 

 country they ceased at a critical stage. The early observers in the 

 middle of the last century became aware of the existence of the 

 variations, and, while experimenting on the deflection of the plumb-line 



