ORIGIN OF CERTAIN CLAYS. 99 



Sandstones sometimes contain a large amount of brown oxide of 

 iron. This is due to infiltration. Occasionally sandstones contain 

 beds of chert, as in the Upper Greensand of Devonshire, Dorsetshire, 

 and the Isle of Wight, and in the Lower Greensand of Sevenoaks ; 

 but since chert is only developed where the sandstone is calcareous, 

 and is nothing but flint more or less porous, which contains some 

 calcareous matter, its origin will be better considered in discussing the 

 flints which are found in limestones. Small masses of phosphate of 

 lime sometimes abound in sands, especially the Upper Greensand of 

 the Isle of Wight and the Neocomian Sands of Bedfordshire and 

 Cambridgeshire, but since they are more characteristic of deposits 

 which have a floor of clay, they will be treated of after discussing the 

 history of clays. 



Clay. 



No such careful and detailed examination has been made of exist- 

 ing mud and clay as of sand or limestone. The subject is much more 

 difficult, and as a rule, nothing can be distinguished by the microscope 

 but more or less irregular granules, minute flakes of mica, and sometimes 

 needle-like prisms, with variable amounts of calcareous granules and 

 sand. There is necessarily every gradation between sands and clays 

 on the one hand, and limestones and clays on the other; and the 

 observations on the deposits now forming are too few to completely 

 demonstrate the conditions under which many of the newer clay beds 

 were formed. It may, however, be regarded as certain, that when the 

 quartz grains in clays are coarse the clays are derived from granite, 

 while when fine they are due to the destruction of schists. The 

 newer clays, as a rule, give no indications of pumice or volcanic dust, 

 but many of the older muds now changed into slate rocks appear to be 

 entirely of volcanic origin. Mr. Sorby has noticed that fine-grained 

 mud obtained in the South Pacific from a depth of 2600 fathoms, 

 possesses the following remarkable property : The grains of sand do 

 not separate from the finer mud and subside, but gather the finer 

 particles about them into a compound granule, and this process rapidly 

 clears the water. But it has been determined experimentally, that the 

 solid matter in such muds only amounts to n per cent., while in 

 shales the solid matter is at least 75 per cent., so that when pressure 

 squeezes the water out of these clays they may be reduced to one-sixth 

 of their original thickness, and this change would tend to develop in 

 the planes of bedding exactly such a fissile structure as is commonly 

 met with. Clay may originate in many ways ; the red earth found in 

 caves, and washed in by the streams flowing through them, is obtained 

 from the destruction of the neighbouring limestone rocks, for after the 

 carbonic acid gas dissolved in water has carried away the whole of 

 the carbonate of lime, there remains an insoluble residue of silicate of 

 alumina and oxide of iron, which, although forming but a small per- 



ftage of the limestone, yet has often contributed to the accumulation 

 small deposits, such as those in caves. In the exploration of the 

 antic by the " Challenger," it was observed that there is much 



