- 3 ~ 



deposition the sea-floor may become a graveyard 

 wherein are concentrated the shells and skeletons of 

 dead organisms. Enclosed in a sarcophagus of 

 phosphate the fossil has enhanced chances of preser- 

 vation. In loosely compacted strata fossils are often 

 dissolved away, and the occurrence of phosphorite 

 beds in such strata may alone provide the means of 

 working out the life sequence, which in turn gives 

 the key to the dating and correlating of the rocks. 



Investigation of the modern sea-floor shows that 

 deposits of nodular phosphorite are especially 

 characteristic of the continental herders of the great 

 ocean "basins and that they occur in the greatest 

 abundance where currents of different temperature and 

 salinity intermingle. The chemical processes leading 

 to the formation of phosphorite nodules, however, are 

 not fully understood. Phosphorus liberated loy 

 weathering of the rocks on land finds its way in 

 solution to the sea. Here it is taken cut of solution 

 largely by organic agencies, vertebrates, brachiopods^ 

 Crustacea and other forms of life contributing to 

 this work. With death and decay of the organisms 

 phosphorus is again released and under conditions of 

 normal sedimentation is dispersed throughout the 

 accumulating detritus « Pauses in sedimentation permit 

 phosphoric enrichment of bottom waters, and chemical 

 interaction in which ammonia is thought to play a 

 part, may produce phosphatic compounds in the form of 

 a gelatinous precipitate, which on hardening cements 

 patches of the sea-floor and the debris thereon. 



The Lower Greensand and Gault formations were 

 laid down in fairly shallow water on a subsiding sea- 

 floor. There were, however periodic halts in the 

 subsidence of the sea-floor and in the deposition of 

 sediment, while the rate of accumulation of organic 

 decomposition products was presumably undiminished. 

 These missing or epitomised chapters in the 

 sedimentary record are today marked by beds of 

 phosphorite nodules. 



