384 Mr. J. F. Walker on a Phosphatic Deposit 



amount of alumina with the fluorine and magnesia, in one ana- 

 lysis, is 6'64 per cent. ; of course, if a special analysis of the 

 nodules and phosphatic casts (the adhering sand having been 

 carefully removed) were made, the percentage of alumina would 

 doubtless be greater. This would indicate that the phosphatic 

 nodules had been formed of clay soaked in decomposing animal 

 and vegetable matter, since the alumina could not be derived 

 from either animal or vegetable sources. 



The question now to be considered is, whether all the remains 

 of organic life found in this deposit are coeval with the deposi- 

 tion of the bed. 



Mr. Seeley states that he has never obtained from this bed a 

 fossil that is extraneous, and that they all appear to him to be 

 " denizens of the old sea-bed where they abound.'^ 



There are obtained from this deposit large masses of silicified 

 wood resembling those found in the Purbeck, also small pieces 

 of wood mineralized with phosphoric acid, and often bored by a 

 new species of Pholas, which I have named Pholas Dallasii. 



It seems improbable that wood existing in two such difl'erent 

 conditions should have been derived from the same source^. 



Amongst the remains of animal origin we find rolled bones 

 and teeth of reptiles and fishes, also shells of MoUusca, existing 

 (as before mentioned) in two distinct conditions. 



The phosphatic casts of shells are generally so much worn 

 that it is impossible to identify their species with precision. In 

 their general aspect they resemble those of the Kimmeridge and 

 Oxford Clays. They consist of casts of Rhynchonella, of Car- 

 dium, Area, Pholadomya, &c., of Pleurotomaria, Chemnitzia, 

 Natica, &c. ; three or four species of Ammonites occur, of 

 which Ammonites hiplex is found in great abundance; several of 

 the Ammonites retain their nacreous lustre. Phragmacones of 

 large Belemnites have also been found. 



Part of the ferruginous shells also appear to have been derived 

 from extraneous sources : amongst these I have obtained a spe- 

 cimen of Exogyra virgula and numerous specimens of Gryphaa 

 dilatata. These shells, on account of their shape, could not 

 contain phosphatic mud when they were deposited. They are 

 in a bad state of preservation, which may be due to two causes, 

 — first, to their having been rolled; and secondly, to their having 

 at the time of their deposition lost part of their animal matter; 

 therefore the removal of their calcic carbonate would be more 

 rapid, and its replacement by the ferric oxide less perfect. 



The other ferruginous shells appear to be of the age of the 



* Since this paper was read, I have obtained a fine specimen of a cone 

 probably belonging to a Cycadaceous plant of the Wealden age. Its 

 length is 26 inches, and its circumference is 2"75 inches. (Pl.XIIl. fig. 5.) 



