W. M. Hutchings — The Origin of some Slates. 317 



combinations, which result, among other things, in the formation of 

 new mica, with the separation of titanic acid in the form of rutile. 



Into these reactions, whatever may be their exact course, even the 

 muscovite in very fine state of division appears to enter ; and there 

 is good reason to conclude that in fine-grained sediments of suitable 

 composition, exposed long enough to the necessary conditions as 

 to pressure, temperature and percolation of solutions, an almost 

 complete regeneration of the " paste " to mica can and does take 

 place, and that this regenerated material, under intenser dynamo- 

 metamorphic action, is converted into some of the forms of micaceous 

 slates known to us. 



The mica so formed is probably what in its more advanced stages 

 of development is often known as " sericite." This is usually stated 

 to be only a special form of muscovite, but the term is really rather 

 vague. Eosenbusch points out that some of the " sericites " of the 

 phyllites show strikingly small optic-axial angles, and that probably 

 more than one substance is included in the term. This would 

 follow naturally from the mode of origin seen in these clays and 

 shales, the proportions of the various ingredients of the deposits 

 being liable to vary considerably. We are still very ignorant as to 

 the exact composition of the micas, and the relationships of their 

 optical and chemical properties. 



Whether we call this newly-formed micaceous mineral sericite, 1 

 or leave it nameless, we may take it to be highly hydrated, and it 

 seems justifiable to assume that it contains magnesia as part of its 

 constitution. In these fireclaj'S, though no ti'ace of biotite may be 

 visible to the most careful microscopic search, magnesia is nearly 

 always present, and usually to an extent which would answer to 

 a considerable percentage of biotite of the kind usual in granites. 



Thus, in a list of analyses quoted by Percy, magnesia ranges up 

 to 1*21 per cent., and a fair average may be given as about 075 per 

 cent. For a comparison we may place this against analyses of 

 granite, taking for instance those given of British granites by Teall 

 (British Petrography), where the maximum of magnesia is 248 

 per cent., the Skiddaw granite containing 1'08 per cent. One fire- 

 clay examined by me, with no trace of visible biotite, contained, by 

 most exact determination, 0-95 per cent magnesia. 



If we now come to consider some of the older slates, — the roofing- 

 slates of Wales and Cornwall, — it seems that much of what we have 

 been able to observe in progress in the clays and soft shales of the 

 Coal-measures finds considerable application, and throws light on 

 some important points. Thus, not one of these slates which I have 

 studied contains any trace of original biotite ; and most of the Welsh, 

 and all the Cornish (from Tintagel, Delabole and district) contain 

 varying, and mostly large, amounts of rutile, in just the same form 

 and mode of dissemination as the fireclays. 



1 Although " sericite " is now stated to be only a form of muscovite, and the use 

 of the term is open to some objections, it will still be well to follow Reuard, who 

 elects to continue its employment when writing of mica having the special physical 

 characters which it shows in the class of rocks now under discussion. 



