286 NATURAL SCIENCE. j„^^. 



Chalk, characterised by certain fossils. On the Continent this 

 subdivision of the Upper Chalk has been carried out with some 

 completeness, but in England the boundaries of these zones are too 

 indefinite for use by the geological map-maker. So far, therefore, 

 as maps are concerned, we shall have to depend on the Chalk Rock 

 as our principal guide in estimating our position in the Upper Chalk. 

 At Taplow, the phosphatic zone is believed to lie about 250 feet 

 above the Chalk Rock. 



The much debated question as to the origin of the phosphatic 

 chalk has not received much light from the most recent communica- 

 tions. We are asked to believe that the phosphoric acid rose from 

 the rocks below the Chalk through hypothetical fissures, along anti- 

 clines which have not been proved to exist. Why, if the acid rose 

 from below, should portions of the Upper Chalk have been acted on, 

 and all the lower parts of the formation left unaltered ? Nor, on the 

 other hand, has any connection between the Tertiary beds and the 

 phosphatised zones been established. At Taplow, twenty feet of 

 ordinary non-phosphatic chalk intervenes between the Tertiary 

 base and the highest phosphatic beds. We are driven, therefore, 

 to the conclusion that the phosphoric acid is one of the original 

 constituents of the rock in which it now occurs. We may infer, 

 moreover, from the fact that foraminifera in the same condition of 

 mineralisation as those which compose the phosphatic rock have 

 been detected in flints in some of the foreign deposits, that the shells 

 were phosphatised while they lay at the bottom of the chalk-sea, and 

 probably soon after the death of the occupants. 



MM. Renard and Cornet, in their exhaustive paper on the 

 Continental phosphatic chalks (3), remark that the mode of occur- 

 rence of the granules points to their having been accumulated under 

 the action of currents. The minute fragments of scales and bones, 

 being of the same material and of about the same size as the 

 phosphatised foraminifera, would be carried with them. The Chalk, 

 in their opinion, is not a deep-sea deposit, but a sediment laid down 

 in comparatively shallow water at no great distance from land ; and 

 these granules are such as could be transported a considerable 

 distance by ordinary marine currents. Animal-life occurs in the 

 greatest abundance near shores ; and to the reptiles and fishes of the 

 littoral zone these authors attribute the concentration of phosphatic 

 material, and the subsequent phosphatisation of the microscopic 

 organisms with which it was mingled. 



The Taplow Chalk also is clearly a sediment such as might have 

 been distributed by marine currents. To the naked eye, indeed, it 

 much resembles a somewhat chalky sand. The theory that it was 

 drifted from a distance to its present situation explains also the fact 

 that the larger organisms, such as Belemnites and fragments of 

 Inoceramiis-sheW, though embedded amongst phosphatic granules, are 

 not themselves phosphatised. Yet the characteristically-shaped 



