JDr. A. Hubert Cox — South Staffordshire Fire-clays. 57 



selected on account of their importance for certain industrial uses, 

 and not on account of any special similarity to those argillaceous 

 rocks usually affected by contact-metamorpliism. They are there- 

 fore clays of a special type with certain chemical characteristics that 

 distinguish them from the more commonly occurring, and therefore 

 more common]}^ metamorphosed, argillaceous rocks. Nevertheless, 

 the iitliological correspondence is close enough to institute certain 

 comparisons. 



Before proceeding, however, to discuss such comparisons of the 

 artificially induced changes with those occurring during contact- 

 metamorphism, the characters of the raw material must be briefly 

 described. The clays subjected to experiment were mostly fire-clays 

 from the Goal-measures of South Staffordshire. The mode of 

 occurrence of the origin of the South Staffordshire fire-clays has 

 been recently discussed by Professor Boulton.* Fire-clays differ, of 

 course, from the commonly occurring clays in their very low alkali- 

 content, and as a rule also in their lower iron-content. The 

 particular clays examined were found to consist essentially of 

 a mixture of two types of material, (1) a fine-grained base or clay- 

 substance and (2) a larger or smaller amount of arenaceous material. 



1. The clay-substance consists of an extremely fine-grained 

 material ; its investigation therefore presents many diflSculties. It 

 is only transparent in the thinnest sections, and is then seen to be 

 pale-yellow or almost colourless, the colour gradually increasing 

 with the thickness, so that the material soon becomes yellowish- 

 brown and almost opaque. It is not quite clear whether the colour 

 is a property of the material itself, or arises as the result of iron- 

 staining. Wherever it could be tested the material proved to be 

 birefringent. In consequence, however, of the very small size of the 

 crystals compensation occurs in any but the thinnest sections, so that 

 the material appears to be almost isotropic. By use of the gypsum 

 plate, however, the double refraction can be made apparent. The 

 clay material appears, therefore, to be entirely crystalline. It has 

 often been supposed that the ultimate base in many clays is an 

 amorphous material, but certainly none of the South Staffordshire 

 fire-clays examined by the writer showed any clear signs of the 

 presence of such matter. Further, no amorphous material could be 

 isolated by means of heavy solutions. When tested in bromoform 

 solutions of varying density it was found that no material floated in 

 solutions of specific gravity less than 2'50. On gradually increasing 

 the density of the solution above that point a slight scum rose to the 

 surface, but even this proved to be crystalline. There is, therefore, 

 an entire absence from these clays of material with a specific gravity 

 less than 2*50. This fact tells strongly against the presence of any 

 amorphous material, seeing that, whei'eas (crystalline) kaolinite has 

 the specific gravity 2"65, the amorphous forms of liydrated aluminium 

 silicate, such as halloysite, have a density of only about 2'0-2'2. 

 If therefore any such substance occurred in the clay its presence 

 should be easy to determine. 



' Trans. Eng. Ceramic Soc, vol. xvi, p. 237, 1916-17. 



