Mr. H. G. Seeley on the Potton Sands. 25 



by this diagram of the succession of sands in this part of our 

 series of strata : — 



North. Red Kock, i.e. Upper Q-reensand. 



Gault. 

 anklin Sands. 

 'Weald Clay. 

 Hastings Sands. 

 Purbeck. 

 Portland Rock, with sand. 



Portland Sand. 

 Kimnieridge Clay (South). 



In the south the sands pass insensibly down into the Kini- 

 meridge Clay, in the north they rise insensibly up into the 

 Upper Greensand ; and the further one travels from the eleva- 

 tion of the Purbeck-Wealden area, the more thoroughly do 

 those and all the cognate beds become represented by marine 

 sands. 



IV. What I meant by the deposit reproducing earlier in time 

 the conditions of the Cambridge Greensand is not what our 

 author is at such pains to show (that the Potton bed is sand, 

 and does not effervesce with hydrochloric acid, while the Cam- 

 bridge bed is a marl which does effervesce with hydrochloric 

 acid), but that both were formed on a long low shore during a 

 protracted period of time, that both derived their phosphoric 

 acid from the growth and decay of sea-weed, that both were 

 open to the actions which furnished the Greensand with its 

 wonderful erratics*. 



V. Our author then reminds us that in one analysis of a sample 

 from this Potton phosphate bed there was as much as 6" 64 per 

 cent, of alumina, magnesia, and fluorine, and adds, " this would 

 indicate that the phosphatic nodules had been formed of clay 

 soaked in decomposing animal and vegetable matter." The au- 

 thor does not tell us whether this has been determined by ex- 

 periment or evolved by some other method ; but it is certainly 

 a notable discovery that by soaking six or seven parts of alumina 

 in decomposing animal and vegetable matter till they increase 

 to 100, you will produce a nodule of phosphate of lime. What, 

 meanwhile, would become of the clay, or in what reservoir all 

 this soaking was to be done, are matters as to which we are left 

 in ignorance. 



VI. I am then criticised for saying that I had gathered no 

 extraneous fossils from the bed. This, with diffidence, on ac- 

 count of the state of the specimens, I still repeat. And it is 

 one of those things which have surprised me most ; for I have 



* See Geol. Mag. July 1866, " On the Cambridge Greensand." 



