in the Strata of the Earth. 



521 



and Wood bridge have been and are still collecting- similar pebbles 

 from superficial beds of gravel of the crag" formation, varying 

 in thickness from one foot to many feet, and extending over 

 areas of variable extent and irregular forms, modified by the 

 sweep of currents, by which the bottom of the tertiary sea was 

 affected during the formation of the crag. 



The contents of these crag gravel-beds are of three kinds, — 



1. Sdiceous sand, and rolled chalk-flints and miscellaneous gravel. 



2. Marine shells, and rolled bones and rolled teeth of land 

 quadrupeds and fishes. 3. Rolled pebbles, resembling cropolites, 

 among many thousands of which in the collections of Professor 

 Henslow and Professor Solly, I have never found one that has 

 just pretensions to that name and title ; although, from this acci- 

 dental fictitious resemblance, the name of coprolite manure has 

 obtained an agricultural and commercial currency, which now 

 cannot be withdrawn, but to which the name of pseudo- coprolite 

 would be more appropriate ; better still would be the name of 

 phosphorite, which I shall use in the following observations. 



Mr. Lawes has established, on the east bank of Deptford creek, 

 near Greenwich, very extensive works for grinding to powder these 

 false coprolites or phosphates, and my attention will be directed — 

 1. To the origin of the pebbles, and the causes which have 

 charged them with phosphoric compounds. 2. To the causes 

 that have dispersed similar phosphoric compounds through nearly 

 all rocks and soils. 3. To the possibility of imitating the natural 

 processes that co-operated to their production, and of converting 

 the valuable phosphates of our sewers to the manufacture of a 

 similar manure, by placing sewage-water in conditions analogous 

 to those which attended the formation of phosphates in the crag, 

 and in other strata formed at the bottom of ancient seas and lakes. 



We may here observe that the presence of peroxide of iron, 

 which pervades all these pebbles, and incrusts or pervades also 

 all the rolled fragments of bones and teeth, and the shells of the 

 crag formation, may be an accident not essential to the production 

 of the phosphorites, though possibly auxiliary to or connected 

 with it. 



I believe the essential condition was the admixture (in a fluid 

 or semi-fluid state) of all the now consolidated ingredients of the 

 marlstone and of the septaria (which we use to make our Roman 

 cement), viz., of clay, carbonate of lime, and protoxide of iron, in 

 a state of mud on the sea bottom, at the time of the disengage- 

 ment of phosphoric compounds from the dung and putrefying 

 bodies of fishes, and of molluscous animals and marine worms, 

 at the bottom of the seas in which the deposition of the London 

 clay was going on. These conditions that attended the depo- 

 sition of the material of the London clay, were similar to those 

 attending the deposits of sedimentary mud (subsequently con- 



