104 CRETACEOUS AND TERTIARY PHOSPHATE BEDS. 



fossils, which, indicate considerable denudation of the underlying 

 Secondary strata and more ancient rocks. The concretions are yellowish 

 brown, of the average size of a chestnut to an apple ; the surface is 

 sometimes smooth and sometimes irregular. The bones of extinct 

 reptiles which occur in the deposit are always mineralised with phos- 

 phate of lime. The bed is rarely more than a foot or two in thickness. 



In the Gault there are several beds which contain concretions of 

 phosphate of lime ; first at the base in the zone of Ammonites inter- 

 ruptus ; secondly, a thin band an inch thick in the lowest. zone of 

 Ammonites auritus ; thirdly, in the zone of Ammonites De-la Ruei. 

 A thicker bed, ten inches thick, occurs in the zone of Ammonites 

 mammilaris at the junction of the Folkstone beds and Gault, and also 

 at the junction of the Upper and Lower Gault, and other bands are 

 found higher up. In some localities, as at Farnham in Surrey, and 

 Puttenham in Buckingham, the bed of phosphatic nodules is sufficiently 

 thick to be worked for the manufacture of super-phosphate of lime. 

 On the opposite coast of France, at Wissant, the nodules of phosphate 

 of lime in the Gault are particularly abundant. 



The middle division of the Hunstanton limestone, in Norfolk, has 

 a concretionary character, but the quantity of phosphate of lime is 

 only about 10 per cent. 



The Upper Greensand is particularly rich in concretions of phos- 

 phate of lime in Cambridgeshire, Bedfordshire, and adjacent districts. 

 The nodules are a very dark brown, sometimes tinged green exter- 

 nally with glauconite, grains of which abound in the marl in which 

 they are embedded. The nodules are extremely irregular in form and 

 rarely more than three or four inches in diameter, though rolled 

 masses are sometimes a foot long. Almost all the organisms are more 

 or less mineralised with phosphate of lime. This deposit rests on the 

 Gault. 



Beds of phosphate of lime occur at or near the base of both the 

 Coralline and Red Crag where those beds rest upon the London Clay. 

 The bed is usually a foot or two in thickness ; the nodules are usually 

 more or less smooth and sometimes polished, of a reddish-brown 

 colour. Among them occur many fossils derived from middle Tertiary 

 beds and London Clay, with numerous contemporary remains of 

 vertebrate animals. There has always been some difficulty in account- 

 ing for the existence of these deposits. It is well known that the 

 island of Sombrero in the West Indies has the limestone rock so 

 mineralised with phosphates as to be quarried and exported for the 

 manufacture of artificial manure. Similar deposits occur at Cura9oa, but 

 we have no evidence as to whether this condition is entirely due to infil- 

 tration from guano deposited on the rocks, or to the former growth and 

 decay of plants like those of the Gulf Weed over a submerged region. 

 Certainly, no such condition could be appealed to in explanation of 

 the origin of the concretionary deposits under consideration, nor would 

 it bring us any nearer to the origin of the material to attribute it to 

 the denudation of deposits in pre-existing Palaeozoic rocks, such as are 

 known to occur in Estremadura. All the phosphoric acid must be 



