Andeew. — On the Clarendon Phosphate-deposits. 473 



5 



(4.) That they were formed in situ by deposition of P,0 

 from descending waters which derived their P 2 5 from the 

 basalt. 



(5.) That they were formed in situ by deposition of P 2 5 

 from percolating waters which derived their P 2 5 from the 

 limestone. 



(6.) That they were formed in situ by the concentration 

 of the phosphatic contents of the limestone — a process due to 

 ordinary weathering and the action of meteoric waters. 



(7.) That they were formed by the combined action of the 

 methods outlined in the last two theories. This combination 

 theory is the one I see most reason to support. 



1. Transportation Theory. — This is extremely improbable. 

 The phosphate boulders are separated from the limestone and 

 from each other by bands of phosphatic clay and by layers 

 of sand. Punning water would most probably have been the 

 agency by which the boulders were transported, if they were 

 transported at all. It is scarcely possible that a violent stream 

 capable of moving a boulder of 2 or 3 tons weight would deposit 

 that boulder on a thin stratum of clay or sand. Much more 

 likely is it that the stream would sweep away clay and sand 

 and deposit the boulders on the bare surface of the limestone. 

 I have not observed a single instance of this direct contact 

 of limestone and rock-phosphate ; I have always found a layer 

 of clay or sand between the two. Moreover, the presence of 

 basalt-fragments imbedded in the phosphate at Kiln Point is 

 sufficient to disprove the theory, for it shows that the rock- 

 phosphate was formed after, and probably a long time after, 

 the extrusion of the basalt. 



2. Concretionary Theory. — It is not at all probable that the 

 rock-phosphate was formed by concretionary action around 

 some nucleus, in a manner analogous to the coprolites of the 

 Upper and Lower Greensands of England. None of the Clare !i- 

 don nodules have been found to contain organic nuclei, and the 

 sharks' teeth and other organic fragments at Round Hill, &c, 

 always lie on the clays, and not imbedded in the hard nodules. 



3. Ascension Theory. — The occurrence of phosphorite and 

 perhaps staffelite as stalactites and as incrustations on the 

 walls of cavities suggests that an aqueous solution of phosphoric 

 acid must have played, and must still be playing, some part 

 in the formation of the phosphate. Could this solution have 

 risen from below % No, it could not, for in many places the 

 phosphate rests on the surface of unaltered limestone ; had 

 the solution risen from below it would first have acted on the 

 lower layers of limestone, converting the calcium-carbonate to 



