of the Chalk in the London Basin. 301 



zone here. Again, fortunately we find two other conveniently 

 situated borings supplying information which can be combined with 

 these figures. In the well-known deep boring at Richmond Water- 

 works the Lower Chalk was 220 feet thick, the Middle Chalk 

 145 feet, the Chalk Kock 5 feet, and the Upper Chalk 300 feet. 

 Adopting the figure of 50 feet for the thickness of the If. cor- 

 testudinariiim zone, it is further necessary to take a figure for the 

 thickness of the M. coranguiniim zone at llichraond. Bearing in 

 mind the recorded thickness of 280 feet for this zone further east in 

 Kent, the writer adopted as a working figure 250 feet for the 

 thickness of M. cura^iguinuvi Chalk at Hichmond. Hence, at the 

 latter site, the level of 670 feet above the base of the formation, for 

 the base of the Marsupites zone, was arrived at. The third boring 

 site affording information is that at Bushey (Colne Valley Water- 

 works), where the Lower Chalk was 255 feet thick, the Middle Chalk 

 267 feet, the Chalk Rock 8 feet, and the Upper Chalk 156 feet. In 

 the absence of any evidence to the contrary the thicknesses of 50 feet 

 for the M. cortestudinarium zone and 220 feet for the M. corang^iinum 

 zone may be taken, and the level of 800 feet above the base of the 

 formation for the base of the Marsupites zone is supplied. Having 

 now three suitably situated working levels for the base of the 

 Marsupites zone, and the present map, the plotting of the concealed 

 boundary between M. coranguinnm and Marsupites Chalk becomes 

 a simple problem in geometry. The result is indicated on the map. 

 In view of the possibility that the figure taken for the thickness of 

 M. coranguinum Chalk at Richmond (250 feet) may be a little 

 excessive, the boundary perhaps lies a little further south between 

 Winkfield and Richmond than is represented. 



Mr. Osborne White gives 70 feet as the superior limit of thickness 

 of the Marsiqntes zone in Berks and Bucks, and the position of the 

 upper limiting boundary of the zone is inserted on the map in 

 accordance with this figure. On the assumption of an average 

 thickness of 25 feet for the Uintacrinus subzone the line of division 

 of the latter from the Marstipites band can also be inserted. 



Field-workers, in referring to the interesting development of 

 phosphatic Chalk of the Marsupites zone, seen at Taplow, liave stated 

 that it is not improbable that the poorly exposed upper part of the 

 section belongs to the succeeding zone of Actinocamax quadratus, 

 although the latter has so far been identified in only two places, 

 namely, Borough Hill, west of Winterbourne, and Kintbuiy, both 

 these places being well to the west of the Taplow-Beaconsfield 

 district. Our present map strongly suggests the existence of the 

 A. quadratus zone in the latter area, and is an interesting incentive 

 to field-workers in that district. From the figures already adopted 

 it will be seen that a thickness of 736 feet accounts for the whole of 

 the Chalk in this area from the base of the formation to the top of 

 the Marsupites zone, and this after giving to the latter its maximum 

 thickness of 70 feet. Consequently it follows that in the neighbour- 

 hood of Beaconsfield there is still a thickness of rather more than 

 200 feet of Chalk to be accounted for, and hence it appears practically 

 certain that there the A. quadratus zone possesses a good thickness. 



