466 ME. A. STEAHAN ON A PHOSPHATIC CHALK [Aug. 1 896, 



iforaminifera, silicified, and not easily distinguishable from the 

 matrix. Some, however, have been filled with a brown mineral 

 resembling the phosphate of the phosphatic chalk; in such the 

 shell only has been silicified, the carbonate having been replaced 

 more readily than the phosphate of lime, as might have been 

 expected. 



The existence of these two stratigraphically distinct deposits 

 throws additional light on the origin of phosphatic chalks generally, 

 for it enables us to select with greater certainty the phenomena 

 which accompany the formation of such a rock. The characteristics 

 which are common to both may be enumerated as follows : — 



J.. The abundance of organic remains (pellets, teeth, and bone- 

 fragments), which originally contained phosphate of lime. 



2. The phosphatization of organisms (foraminifera and shell- 

 prisms) which were originally composed of carbonate of lime. 



.3. The impersistent character of the deposits. 



4. The association of the deposits with a floor of hard nodular 



chalk. 



5. The perforation of the white chalk beneath this floor by 



branching tubes filled with phosphatic chalk. 



1, 2, & 3. On the first two of these points I dwelt at some length 

 in my former paper ; the impersistent character of the deposits 

 which was suggested by the non-appearance of the Taplow phos- 

 phatic Chalk elsewhere seems now to be confirmed by the fact that 

 the Southerham Chalk thins away or passes into ordinary white 

 chalk within a few yards. The Continental deposits, moreover, 

 which have been extensively worked, form lenticles rarely exceeding 

 1 kilometre in length and about 200 or 300 metres in breadth. 



4. But the association of the deposit with a hard floor and the 

 perforation of the chalk beneath by tubes filled with phosphatic 

 chalk acquire some significance from being repeated in a different 

 locality and at a different horizon. 



Such floors are far from uncommon. One of the best known and 

 most extensive is that known as the Chalk Rock, but they occur 

 also at all horizons in the Upper and Middle Chalk. They are 

 ispecially characterized by an unusual hardness and a nodular or 

 lumpy structure ; by the presence of glauconite either as casts of 

 microscopic organisms or as coating the nodules ; and by a slight 

 increase in the proportion of phosphate of lime either disseminated 

 through the rock or filling the small organisms. 



The unusual hardness of the floors on which the phosphatic chalks 

 of Taplow and Southerham rest suggested partial phosphatization 

 .of the rock. This, however, was disproved by an analysis by 

 Mr. Player which showed the presence of only a very small quantity 

 of phosphoric acid; nor was silica or magnesia present in sufficient 

 quantity to account for the character of the rock, the composition 

 of which, in fact, scarcely differs from that of ordinary chalk. 



The existence of calcite in veins and cavities is an unusual feature, 

 &nd suggested that the hardness might be due to a crystalline 



