570 STRATIGKAPHICAL GEOLOGY 



the first because the white clays occur at a considerable distance 

 from the nearest areas of granite. It is true that there may have 

 been granite masses not far south of the Isle of Wight, as Mr. 0.. 

 Fisher has pointed out, and there may have been others between 

 Normandy and Devonshire ; still none of them are likely to 

 have been large enough to provide so much pure argillaceous 

 material. Again, with respect to the Chalk, the Eocene deposits 

 rest nearly everywhere on Upper Chalk, which contains a very 

 small percentage of clay. The Chalk-marl could have supplied a 

 larger amount, but the contemporaneous outcrops must also have 

 been some distance from the areas in which the Eocene Clays were 

 being deposited, and a very large mass of Chalk-marl would be 

 required to furnish the extensive beds of white clay that seem to> 

 have been formed. Much of the insoluble residue obtained from 

 marly chalks is colloid silica, and the amount of alumina is 

 generally small, the amount of argillaceous matter varying from 

 6 to 20 per cent, and much of this is glauconite, not clay. 



Lastly, there seems no good reason for supposing that the white 

 clays had a different source from the variegated clays of the 

 Reading Beds or of the Oligocene Series. So far as is known, they 

 only differ in colour, and this, as Dr. Irving points out, may be 

 due entirely to the leaching action of peaty acids which would 

 operate on clay derived from any source. He has also shown that 

 the green colouring matter of certain Eocene sands and earths is 

 due to decomposed vegetable matter, while we know that 

 variegated white and red clays are common among lacustrine 

 deposits of many ages all over the world. 



It seems probable, therefore, that these fine Tertiary Clays are 

 merely the fine muds brought down .by rivers and streams from all 

 parts of the country which surrounded the areas of deposition, and 

 that their colour or want of colour is the result of the chemical 

 actions to which they were exposed during their transportation 

 and during the process of deposition, the red tints being due to the 

 oxidation of iron or manganese, the green to the presence of 

 decomposed vegetation, and the white to leaching out of all 

 colouring matter. 



With respect to the other prevalent kind of deposit mentioned 

 on p. 569, that is more easily accounted for. The foraminiferal 

 limestones are specially characteristic of the Mediterranean area of 

 deposition, where they constitute masses of limestone many hundred 

 feet in thickness. The ordinary " Nummulite limestone " has a 

 pale buff or grey matrix, composed mainly of the debris of 

 decayed and disintegrated Nummulites, in which are embedded 

 more perfect tests of one or more species. The genera Nummulites, 



