383 



Mr. J. F. Walker on a Phosphatic Deposit 



At a coprolite-working on the left side of the line, looking 

 towards Cambridge, a few yards from the edge of the cutting, 

 the bed increases in thickness to 2 feet. 



At a large working on the hill the conglomerate bed is about 

 6 feet thick ; the section is as follows : — 



1. Sandstone, on which conglomerate rests. 



2. Conglomerate bed, 6 feet. 



3. Flaggy sandstone, not exceeding 1 foot in thickness (often less), and 



surface soil. 



The lower part of the conglomerate is here darker in colour 

 and more indurated than the upper. 



On the other side of the road is another working, where the 

 nodules lie in loose sand, and the phosphatic bed is about 

 1 foot thick. 



There are several other workings in the neighbourhood. 



The conglomerate contains phosphatic nodules and pebbles in 

 about equal proportions. The bed is dug out and sifted, washed 

 and laid in heaps, and then conveyed into sheds, where the no- 

 dules are picked out by hand. The quantity of phosphoric acid 

 in the nodules varies from fifteen to twenty-two per cent. 



It seems to be the opinion of Mr. H. Seeley that this bed is 

 the southern extension of the Carstone, which, in a former 

 paper*, he has stated to represent the Gault and the Shanklin 

 Sands. But the Gault in this district is represented by a clay 

 lying beneath the Upper Greensand. Therefore, if his views 

 be correct, the term Carstone is inapplicable to this bed. 



Mr. Seeley states that this deposit " reproduces earlier in time 

 the conditions of the Cambridge Greensand." 



The Greensand of Cambridge consists of a fine marl which 

 eflfervesces briskly with hydrochloric acid ; it also contains green 

 grains, angular boulders, and hard dark-coloured nodules of 

 phosphatic matter, often covered with Plicatula ; lumps of iron 

 pyrites are occasionally found. All the fossil shells are filled 

 with the same material of which the nodules are composed. 



* Quart. Journ. Geol. Soc. Nov. 1864. 



