96 THE ENVIRONMENT 



upon being transported into areas of sedimentation, may recombine 

 to form clay minerals. 



That is the reason why we now have only three types of sediment: 

 sand, clay and limestone. The sands are exclusively quartz sands, the 

 left-overs of chemical weathering, although perhaps transported and 

 re-sedimented several times. The clays are newly formed by re- 

 combination of ions derived through chemical weathering from sili- 

 cates. The carbonate material of the limestones is mainly of a bio- 

 genic origin, derived from animal shells. 



Only under exceptional circumstances will, for instance, sand still 

 contain an appreciable amount of felspar. This occurs when detritus 

 of igneous rocks is deposited quite near to its source area, and 

 speedily buried in such a way that further chemical weathering is 

 prevented. Sulphides, which do not only weather more quickly, but 

 are also readily attacked through biochemical action by sulphur 

 bacteria, nowadays are found even more rarely in sands. There are 

 some examples from high up in the tundras, where the fiercely cold 

 climate is prohibitive of chemical erosion, or from the rapidly sub- 

 siding Indus valley, where younger sediments, quickly covering older 

 deposits, effectively seal off the air supply and thereby prevent further 

 weathering. These exceptions, by their scarcity, and by the ex- 

 tremeness of the conditions of their environment, only stress the 

 more strongly that normally in our present oxidative atmosphere all 

 sands are quartz sands; the only stable minerals are oxides. 



MINERALS STABLE UNDER ANOXYGENIC ATMOSPHERE 



This will not have been so under a primeval atmosphere of reducing 

 character. There, felspars, dark minerals and sulphides could have 

 lain on the surface of the earth for a much longer time before they 

 finally disintregrated. They even could have been taken up in a new 

 erosion — transportation — sedimentation sequence; for instance, when 

 through slight crustal movements, through a changing of river courses, 

 or lowering of the sea level, their original sedimentation area would 

 become attacked by erosion. They would weather much more through 

 mechanical attack, and much less through chemical weathering. Con- 

 sequently in repeated sequences of erosion — transportation — sedimen- 

 tation, they would become well rounded and get well sorted as to 

 size and specific weight. 



