W. M. Hutehings — Clays, Slates, and Contact- Metamorpliism. 39 



alkali present is combined in tbe very abundant mica, and we can 

 calculate by Tscbermak's formulae, as Renard does in bis slate 

 analyses, tbe alumina tbat would be required to form muscovite, with 

 a proportion of paragonite due to tbe soda. If we do tbis, we find 

 tbat tbe alumina required is respectively : No. 2=19-39 per cent., 

 No. 3=19-54: per cent.. No. 4=:19'15 per cent., No. 5=19-04 per 

 cent., and No. 6=19-29 per cent., leaving only 2-46 per cent., 504 

 per cent., 2-78 per cent., 5-49 per cent., 446 per cent., available for 

 kaoline or any other combinations. 



These figures are on tbe assumption tbat only tbe alkali is present 

 in the mica, requiring alumina in propoi-tion. But it is more than 

 probable, as Renard considers, that part, at least, of tbe magnesia, 

 and perhaps some of the lime, are also combined in tbis mica, in 

 which case a good deal more alumina would be required, though 

 tbis would be partly compensated, possibly, by tbe rephicement of 

 some alumina by ferric oxide. At any rate, these calculations are 

 able to show tbat not very much kaoline can be present in these 

 clays as they now are. And tbe evidence, chemical and microscopical, 

 leads to the belief that, supposing kaoline formed a part of the 

 original deposits, as it probably did, it was either sparingly repre- 

 sented or has been altei-ed since. Tbe same arguments, supported 

 by analj'^ses, apply equally to many slates in which no felspar can be 

 detected, in which mica is tbe only mineral present which contains 

 alkali, and in which very little alumina is available after calculating 

 what is required for tbis mica. 



Of course there is no doubt tbat there are clays, in tbe Coal- 

 measures and elsewhere, whose analyses show tbat a great deal of 

 silicate of alumina is present as such. Some of the " fireclays " are 

 examples of this, owing to removal of alkali by vegetation from tbe 

 original felspar, and perhaps also from the mica originally deposited. 

 And there are slates of similar composition in tbe older formations. 

 They are, however, few in proportion to what may be called normally 

 composed slates. 



Tbe mistake is often made of speaking of claj's as if they were 

 largely, or mainly, composed of tbe chemical waste of felspars, and 

 as if tbis must always be kaoline. As I have previously shown, an 

 examination of any series of the gradations from fine-grained clays 

 into shales and sandstones disproves tbis completely. There is a 

 great deal of felspar present which has been brought down as such. 

 A decomposing felspar will be more easily broken up by denudation, 

 and with the products of decomposition there will be carried away 

 tbe fragments of the mineral itself, together with other felspar 

 fragments not necessarily from decomposing crystals. In the coarser 

 parts of tbe deposits this felspar has partly survived ; in tbe finer 

 mud it has all passed into secondary products. We know, from tbe 

 experiments of Daubree and others, bow rapidly and easily felspar 

 is decomposed when in a very fine state of division. So far as I 

 have been able to follow the subject, tbis decomposition, after deposit 

 in these clays, leads to tbe formation of mica and not of kaoline. 

 Continued attention, which I have paid to the alteration of felspar 



