Yol. 52.] WITH HOLASTEB PLANUS AT LEWES. 469 



seem only in part reconcilable with such a theory as the above, as to 

 its origin. The rocky floors mark the pause or interruption in the 

 sedimentation caused by the change of current, while the probably 

 accompanying change of temperature would help in explaining the 

 presence of the glauconite and phosphates. On the other hand, it 

 should be remembered that the phosphatic deposits bear internal 

 evidence, in the shape of the abundant excreta, of having been a 

 feeding-ground rather than a burial-place. 



5. On the fifth point of resemblance between the Taplow and 

 Southerham sections, namely, the perforation of the underlying white 

 chalk by branching tubes filled with the phosphatized variety, I have 

 found little to throw any additional light. The tubes are rounded, 

 and, being filled with a material markedly different from the sur- 

 rounding rock, are well-defined. 1 The phosphatic chalk in these 

 exactly resembles that of the phosphatic stratum itself ; it, more- 

 over, spreads here and there for a few inches along planes of 

 current-bedding in the white chalk. The tubes range up to an 

 inch in diameter and run vertically as often as in any other direc- 

 tion ; they branch frequently and irregularly. 



These tubes perhaps approach ' Spongia paradoxica ' more nearly 

 than any other structure with which I am acquainted, except in being 

 generally smaller. The origin of this so-called organism remains 

 doubtful after much discussion. Prof. Hughes has argued in favour 

 of the structure being wholly concretionary 2 ; but in the present 

 case, where there is a marked difference between the contents of 

 the tube and the surrounding rock, his arguments would not apply. 

 It seems necessary to suppose that the tubes existed as such, and 

 were open, and that phosphatized organisms were being occasionally 

 washed about, while the white chalk was being deposited. Against 

 their being the borings of molluscs, 3 or casts of the hollows left by 

 seaweeds or annelids, various objections may be urged, and on the 

 whole Zittel's supposition with respect to ' Spongia paradoxical 

 that it is the cast of a horny sponge, seems the most applicable in 

 the present case also. 



Conclusions. 



The close resemblance of the Southerham chalk, at the base of 

 the Upper Chalk, to the Taplow chalk, at the top of that formation, 

 indicates that the conditions under which such a deposit was formed 

 were not confined to any one zone. They may probably have 

 recurred at any horizon in the Chalk. 



The conclusion formed with regard to the Taplow phosphatic 



1 A thin section across one of the tubes and the surrounding matrix was 

 described in my former paper, Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 358. 



2 Quart. Journ. Geol. Soc. vol. xl. (1884) p. 276 and discussion. 



3 M. de Mercey, in describing a nodular chalk which forms the top of the 

 Micraster-zone, speaks of it as being sometimes pierced by Pholad.es. Numerous 

 tubulures penetrate it deeply. Bull. Soc. G60I. France, ser. 2, vol. xx. (1863) 

 p. 635. 



