At Potton, phosphatized nodules may be obtained, and here a hard bed, the "Carstone," lies at the top of the formation.
In the Cambrian limestones, as in their more recent analogues, layers and nodules of chert and phosphatized material are not wanting.
Many of the oceanic islets are composed of coral limestone, which in this way becomes phosphatized; others are igneous, consisting of trachyte or basalt, and these rocks are also phosphatized on their surfaces but are not so valuable, inasmuch as the presence of iron or alumina in any quantity renders them unsuited for the preppration of artificial manures.
The leached guanos and phosphatized rocks, which are grouped with them for commercial purposes, have been obtained in great quantities in many islands of the Pacific Ocean (such as Baker, Howland, Jarvis and McKean Islands) between long.
To these leached guanos and phosphatized limestones the name sombrerite has been given.