Vol. 52.] WITH HOLASTER PLANUS AT LEWES. 467~ 



cement. Under the microscope, however, the rock is seen to 

 consist of an amorphous calcareous paste, enclosing numerous 

 foraminifera, though not nearly in such abundance as the phosphatic 

 chalk. The foraminifera, many of which are perfect, are filled with 

 the same material as that which forms the matrix. No foreign 

 minerals were observed. 



In spite of the negative character of the microscopic evidence, it 

 seemed not unlikely that the replacement of the carbonate of lime 

 (in the organisms which were originally calcareous) by an acid 

 phosphate had set free carbonic acid, and led to the production of 

 a soluble bicarbonate of lime. A solution of this coming into 

 contact with the underlying and less permeable white chalk might 

 have hardened it by infiltration, and might explain also the existence 

 of the calcite in the joints and cavities. 



But it must be remembered that the association of phosphatic 

 chalk with these floors is the exception and not the rule. Though 

 the hardened chalk underlies the phosphatic chalk in both the 

 English sections, and in some of the Continental occurrences also, in 

 the great majority of cases these rock-bands are overlain by white 

 chalk of the normal character. Their lumpy and nodular structure 

 seems then to suggest a concretionary action, probably accompanying 

 a pause or change in the sedimentation ; that the action was contem- 

 poraneous is proved by the fact that small organisms not unfrequently 

 adhere to the nodules. 1 



Before quitting the subject I should mention that in the Chalk 

 Bock there is evidence of something more than a mere change in 

 the sedimentation. It has long been known that the fauna of this 

 horizon is peculiar, especially on account of the presence of certain 

 gasteropoda ; foreign detritus, moreover, becomes more abundant in 

 this band than elsewhere in the Upper Chalk. Dr. Hume, as a 

 result of an exhaustive examination of the residues and conditions 

 of deposit of the Chalk, concludes that ' lithological evidence points 

 to the close of the Middle Chalk period as having been a time of 

 change, probably in the direction of re-elevation. The palaeonto- 

 logical facts seem peculiarly striking in this respect.' Holaster, 

 Ammonites, and Scaphites, which were last met with in abundance 

 in the Grey Chalk, return in the Chalk Bock, and the arenaceous 

 foraminifera, the abundant quartz-grains from the residues, the 

 crystals of tourmaline, were all left behind in the Grey Chalk, but 

 reappear in the Chalk Bock. He shows, too, that the other mollusca 

 also were affected. ' Turbo gemmatus and forms of Trochus and 

 Solarium again resume their place among the important fossils of this 

 series. The trochoid forms are the last to disappear during depression, 

 and they are also the first to reappear during elevation.' 2 



1 Hill & Jukes-Browne, Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 230; 

 see also 'Geology of the Isle of Wight,' 2nd ed. Geol. Surv. Mem. 1889,' 

 pp. 78, 79. 



2 ' Ocean Deposits, Ancient and Modern,' by W. F. Hume, ' Natural 

 Science,' vol. vii. (1895) p. 390; and 'Chemical and Micro-Minera logical 

 Researches on the Upper Cretaceous Zones of the South of England,' 1893. 



