-----------------------------+--------------+----------+-------- DISTURBANCES OF THE STRATA. The last question to be considered relates to the disturbances which may have affected the strata; for whatever may be the absorbent power of the strata, the yield of water will be more or less diminished whenever the channels of communication have suffered break or fracture. If the strata remained continuous and unbroken, we should merely have to ascertain the dimensions and lithological character of the strata in order to determine their water value. But if the strata is broken, the interference with the subterranean transmission of water will be proportionate to the extent of the disturbance. Although the Tertiary formations around London have probably suffered less from the action of disturbing forces than the strata of any other district of the same extent in England, yet they nevertheless now exhibit considerable alterations from their original position. The principal change has been that which, by elevation of the sides or depression of the centre of the district, gave the Tertiary deposits their present trough-shaped form, assuming it not to be the result of original deposition. If no further change had taken place we might have expected to find an uninterrupted communication in the Lower Tertiary strata from their northern outcrop at Hertford to their southern outcrop at Croydon, as well as from Newbury on the west to the sea on the east; and the entire length of 260 miles of outcrop would have contributed to the general supply of water at the centre. But this is far from being the case; several disturbing causes have deranged the regularity of original structure. The principal one has caused a low axis of elevation, or rather a line of flexure running east and west, following nearly the course of the Thames from the Nore to Deptford, and apparently continued thence beyond Windsor. It brings up the chalk at Cliff, Purfleet, Woolwich, and Loampit Hill to varied but moderate elevations above the river level. Between Lewisham and Deptford the chalk disappears below the Tertiary series, and does not come to the surface till we reach the neighbourhood of Windsor and Maidenhead.