W. M. Hutchings — Anh-slales of the Lake- District. 157 



mucli more of its nature. The dark granular matter, enlarging into 

 transparent grains, is still present in the same abundance as before. 

 Its isotropic character is now more easily ascertained. Taking its 

 various characteristics as stated, together with the fact that no 

 amount of boiling in strong acid affects it, there is no reason to 

 doubt that this granular mineral is garnet. It is far too minute to 

 allow of any mechanical separation of it. The brightly-polarizing 

 rods are also present as before, and it may now be made out 

 that they are the transverse sections of minute flakes of a faintly 

 yellowish to greenish mica, apparently a " sericite " in all respects 

 analogous to the minute mica of the fireclays and sedimentary slates. 



This mica and the garnets are seen, especially on rotation in 

 polarized light, to be intimately mixed in with some other substance, 

 whose exact nature cannot be determined. There is a very minute 

 mosaic of low polarization-tints, partly due to such of the mica as 

 lies more or less flat in the field, but in which, one is led to expect 

 also quartz or felspar, or possibly both. More or less of the com- 

 ponent grains of this mosaic show faint dark crosses, due to spheru- 

 litic structure, as of chalcedony. 



A proper quantity of powder of this slate having been carefully 

 prepared in the above manner with acid and caustic potash (especial 

 care being taken to wash out the last traces of the potash), was 

 dried at 110° C. and a very careful analysis has been kindly made 

 for me by my friend Mr. George Paterson, of Liverpool, to whom 

 I am much indebted for the great trouble he has taken in making 

 this and the following analyses for me. 



(Includes a good deal of undeter- 

 mined ferrous oxide, — hence 

 excess in the total.) 



101-22 



A consideration of this analysis shows at once that a large amount 

 of free silica is present. It is, perhaps, not of much use to calculate 

 the mineral-composition, even approximately, from the above figures, 

 because we do not know the constitution of the sericitic mica, which 

 is probably more complex than normal muscovite, and contains 

 lime, magnesia, and iron. 



Assuming, for the sake of argument, that its composition is similar 

 to that of muscovite (Tschermak's formula), and that all the potash 

 of the analysis is in the mica, then the alumina required would be 

 16-06 per cent, as against 15-70 shown by the analysis. But then 

 there is the garnet to provide for, and we have no safe basis on 

 which to calculate its composition. It is probably a lime-iron- 

 alumina garnet. Also there may be felspar present finely diffused 

 in the mosaic. All that we are able to say with safety is, that after 

 removal of the abundant chlorite from this slate we have remaining 



