Oe Pan ake ace a Te =e NOES oe mete TREAD Oot PONE ite pe Dmg oe ge mae ee ne § Sa SP Tae SS 
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 Roek, é.e. Upper Greensand. 
Shanklin Sands. 
Weald Clay. 
Hastings Sands. 
: Purbeck. 
8 Portland Rock, with sand, _ 
°C Portland Sand. 
Kimmeridge Clay (South). 
In the south the sands pass insensibly down into the Kim- 
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.” 
