W. M. Hutchings — The Origin of some Slates. 271 



the minutely-granular " kaoline," and of micaceous material with 

 rutiles. 



It is stated in nearly all books on the subject that clays consist 

 mainly of hydrous silicate of alumina, and this definition is 

 repeated, in works dealing specially with fire-clays, as true of them 

 also. Thus Percy (Metallurgy — volume on Fuel, Fire-clays, etc.), 

 says : " All clays as they occur in nature consist essentially of 

 hydrous silicate of alumina." 



This definition, derived as it is mainly from the chemical study of 

 clays, is certainly not universally true, and is seldom true as to 

 fire-clays. It is, however, very much more true in a chemical sense 

 than in a mineralogical or petrological one. Later on in the same 

 treatise Percy, with more of his usual caution, sums up as follows : 

 "The conclusion, which appears to be justified by the foregoing 

 considerations, is that one definite hydrous silicate of alumina, 

 namely kaoliuite, has been found in many instances to constitute the 

 basis of suhstances usually designated clay ; and in the present state 

 of our knowledge more than this cannot confidently be asserted." 



As regards the presence of kaoline in clays, it appears to me that 

 the word is often used very vaguely. We know, of course, that 

 kaoline, a definite hydrous silicate of alumina, is formed during the 

 decay of felspar, and we can frequently obtain it well crystallized, 

 as in the interstices of many grits and sandstones. In this pure 

 form, however, it rarely, if ever, reaches deposits of clays. China- 

 clay is usually supposed to consist most largely of it ; but deposits 

 of this clay are comparatively rare and limited, and are due, probably, 

 largely to other causes, as well as to the ordinary atmospheric 

 weathering of granites. And even in the purest china-clay the 

 microscope shows the presence of a very large proportion of mica 

 and other things. Moreover, we know that the decay of orthoclase 

 felspar does not give rise only to kaoline, but also to mica; and, as 

 Rosenbusch points out, we are not well able to distinguish micro- 

 scopically between the two processes when in progress, — processes 

 " which are chemically very nearly related, and depend upon a 

 partial or total replacement of potash by water with separation of 

 4Si0. 2 ." It is probable that decaying orthoclase yields to denudation 

 as much very minute hydrated mica as it does kaoline. The 

 latter, in course of transport with water, will become reduced to 

 impalpable granules, will be thoroughly incorporated with the finest 

 waste of other minerals, and will finally be deposited as a very 

 complex substance. 



So far as I am able to make out, the very thinnest possible films 

 of a " fireclay " show under a good |- inch objective a minutely- 

 granular substance, usually of very faint yellowish tinge, and of 

 such extreme tenuity that in polarized light it is either quite inactive 

 or depolarizes only just perceptibly in a faint " speckly " manner. 

 This granular matter is, I suppose, the mixture usually spoken of as 

 kaoline. As soon as we examine a film a little thicker, but still 

 extremely thin, we see between crossed Nicols a more distinct 

 depolarizing action of the granular matter itself, and also the 



