490 LOWER OOLITIC ROCKS OP ENGLAND : 
It may be remarked that Fuller's Earth has been found in other 
formations, especially in the Lower Greensand of Nutfield and "Woburn : 
and in old times in Hampshire, a fact verified by Mr. T. W. Shore. 
Impure Fuller's Earth has been noted also in the Ludlow Beds of Ludlow, 
Buildwas, and Hales End, Malvern, and I have observed it near May 
Hill. Fuller's Earth has also been discovered at Rhiwlas, near Bala. 
Locally thin beds have been found in other formations. 
The question of the origin of Fuller's Earth, is difficult of solution, and 
more experiments are needed in reference to its physical properties. 
Dr. Percy has remarked that the fictile or plastic property of clay 
depends on the presence of the water of combination. All clays contain 
water of absorption or hygroscopic water, which may be expelled without 
lessening their plasticity. Clay heated to redness loses not only its 
hygroscopic water but also its water of combination : in this dehydrated 
state it cannot directly combine with water and regain its plasticity, 
though it may absorb water with avidity. Thus pounded brick may 
absorb water, but regains no plasticity.* 
Messrs. S. "W. Johnson and J. M. Blakef point out that " The plasticity 
of clay is a physical character, and appears to have a close connection 
with the fineness of the particles." It maybe "related to the form of 
the plates of kaolinite. perhaps to their thickness, but this is a subject 
that requires further investigation. Our observations indicate that the 
impurer sedimentary clays are the most plastic * * The plasticity may" 
be, therefore, in part due to the impurities ; " but as remarked by Dr. 
Percy the plasticity must obviously be lessened by the presence of inter- 
mixed non-plastic matter, such as silica. Analyses by Messrs. Johnson 
and Blake show that clays identical in composition may differ notably in 
degree of plasticity. 
Colonel C. W. Pasley mentions that he tried the Fuller's Earth of 
Eeigate mixed with Chalk in the making of artificial cement, and it 
entirely failed. He inferred that plasticity was essential in the clay used 
for cement ; and adds, " But on washing fche Fuller's Earth repeatedly, in 
order to get rid of the small portion of salt, also said to form a part of 
that substance, we found that it became more plastic, and in this state it 
frequently succeeded in forming a water cement, but inferior to that of 
the plastic clays. "J 
Mr. J. Hort Player, to whom I am indebted for analyses of 
Fuller's Earths, tells me that he has " come to the conclusion 
that the choice of the various varieties of Fuller's Earth, for full- 
ing and detergent purposes, has been determined by their physical 
properties as distinguished from their chemical composition. In 
composition they vary at least as much amongst themselves, as 
they differ from clays unsuited to the purpose for which they are 
used. As a matter of fact, they do not absorb oil more readily 
nor to any greater extent than does dried London Clay ; but 
then they fall to pieces more easily in water, they do not become 
plastic, and they are much more cleanly in appearance than 
London Clay. I expect too that from the crude earth, the 
marketable Fuller's Earth is readily prepared, and that whilst other 
argillaceous deposits might yield an equally serviceable detergent 
material, they would not be so easily dealt with." 
* Metallurgy: Refractory Materials, &c., ed. 2, 1875, p. 87. 
t Amer. Journ. Sc., ser. 2, vol. xliii., 1867, pp. 357. 
t Observations on Limes, Calcareous Cements, &c., 1838, p. 57. 
Letter dated June 1st, 1891. 
