“ Three distinct types of this formation are recognized: (1) The Reading type, a series of lenticular mottled clays and sands, here and there with pebbly beds and masses of fine sand converted into quartzite. These beds are generally unfossiliferous. They are found in the N. and W. portions of the London Basin and in the Hampshire Basin. (2) The Woolwich type, grey clays and pale sands, often full of estuarine shells and in places with a well-marked oyster bed. At the base of the shell-bearing clays in S.E. London there are pebble beds and lignitic layers. ”