.e^. PHOSPHATIC CHALK. 287 



prisms, yielded by the complete disintegration of the Inoceramiis-sheW, 

 form separately a large proportion of the phosphatic granules. Hence, 

 we may suppose that phosphatisation took place before the 

 drifting of the materials, and that only those organisms which were 

 originally small, or which had been broken down to the requisite 

 size, were transported. 



On the other hand, we find difficulty in supposing the Taplow 

 Chalk to have been derived by any process of sifting from a coast- 

 sediment. It is believed by many that dry land lay in the West of 

 England during the Chalk Period ; but, even ii this were so, what 

 sort of land can it have been that yielded neither mud nor mineral 

 fragments, both of which are almost entirely absent from the Taplow 

 Chalk ? Even the finer constituent of the rock consists either of 

 minute organisms or of fine powder derived from the breaking up of 

 the larger shells ; and from whatever direction it was drifted, it must 

 have been from a region almost unreached by mineral sediments. 



In considering the effects of a current on these granules we 

 should rem.ember that phosphate of lime has a greater specific gravity 

 than the carbonate, and that, consequently, there would be a tendency 

 for the unaltered granules to separate themselves out from a partially 

 phosphatised assemblage of microscopic organisms. In a current of 

 suitable velocity the phosphatic granules would be naturally concen- 

 trated, much as they are artificially in the washing of the phosphatic 

 chalk for the market. If we suppose some such natural washing as 

 this to have taken place, we need not imagine the granules to have 

 travelled far, and can understand the absence of far-transported 

 mineral fragments. We could also explain the existence of the 

 abundant little coprolites, which have been noticed in the Taplow 

 Chalk, on the supposition that, whether from a set of the tides or 

 other cause, there were banks or hollows in the bottom of the 

 Chalk-sea that were especially frequented by small fishes, to whose 

 efforts was due the phosphatisation of the organic deposits of those 

 particular spots. What fishes these were, and why their actions are 

 observable in the Upper Chalk only, we have not yet ascertained. 



REFERENCES. 



1. Strahan, A. — On a Phosphatic Chalk with Belemnitella quadrata, at Taplow. 



Quart. Journ. Geol. Soc, vol. xlvii., p. 356 (1891). 



2. De Mercey, N. — Remarques sur les gites de phosphate de chaux de la 



Picardie. Bull. Soc. Geol., France, ser 3, vol. xix., p. 854 (1891). 



3. Renard, A. F., and Cornet, J. — Recherches micrographiques sur la Nature 



et rOrigine des Roches phosphatees. Bull. Acad, roy., Belgique, ser. 3, vol. 

 xxi., p. 126 (i8gi). 



A. Strahan. 



