W. M. Hutchings — Ash-slates of the Lake-District. 223 



Among the mica in the larger crystals may often be seen quartz, 

 and usually more or less of perfectly glassy clear felspar. 



Where the alteration has given rise to the pale chloritic mineral, a 

 more definite orientation appears to largely prevail. This chlorite 

 is very faintly dichroic, and polarizes in thin sections in tints up to 

 yellowish-white of the first order. 



Calcite frequently accompanies the other two products, and often 

 occurs by itself, in all stages up to the total replacement of felspar 

 crystals by pseudoraorphs in calcite. 



Mr. Alfred Harker, in a petrological appendix to a paper by Prof. 

 Nicholson and Mr. Marr (" The Cross Fell Inlier," Quart. Journ. 

 Geol. Soc. vol. xlvii. pp. 512-525), alludes to the fact that in some 

 of the rocks in question the evidence goes to show that a large part 

 of the felspars are " regenerated," and have a secondary twinning 

 due to crushing. He specially alludes to a rock from Wythwaite 

 Top, giving details as to his observations on the porphyritic felspars 

 contained in it and their alterations (p. 515). 



Similar occurrences may be noticed more or less all over the Lake 

 District, and these rocks offer a splendid field for the investigation 

 of the many questions, as yet only partially understood, as to the 

 re-generation and re-crystallization of felspars, with or without 

 secondary twinning, due to crushing and shearing. Specimens may 

 be obtained at many points from ashes (fine tuffs and slates), and 

 often from altered lavas, in which the usual turbid felspars full 

 of mica, chlorite, or calcite, with the twinning nearly or wholly 

 obliterated, are replaced by more or less glassy clear crystals often 

 beautifully twinned. These are not crystals and fragments which 

 have escaped decay, but are obviously felspars which are re-formed 

 in situ, — often, apparently, completely re-crystallized and re-twinned. 

 The rocks in which these occur all give full evidence of great 

 stresses, and the felspars themselves are often bent, broken, and 

 re-cemented with chlorite in a most complex manner. 



In the slates of Honister Crag, and many other quarries, any 

 number of bits may be seen, as clear as fragments of window-glass 

 except for a little brownish dust in some cases, a large proportion of 

 such bits showing beautiful twinning in polarized light. Many 

 other bits, equally clear and glassy, show no twinning whatever, 

 nor any cleavage, and are only to be discriminated from quartz by 



microscopically between kaoline and muscovite as alteration-products in decomposing 

 felspars. Rosenbusch, for instance, points tbis out very empbatically (Physiographie 

 der MineraKen, pp. 516, 561). Tbe flaky alteration-product in felspars is certainly 

 sometimes so minute, and occurs in aucb an indistinct manner, tbat decision as to its 

 exact character is very uncertain or impracticable. But as soon as the flakes are 

 larger and more distinct (and this is often eminently the case in the rocks now under 

 consideration), I venture to think tbat tbe identification of tbe mica can be safely 

 made. Tbe bi-refraction of kaolinite is high, but so far as my observations go it is 

 very distinctly less than tbat of muscovite, observing transverse sections in each case. 

 Then again, edge-sections of tbe mica extinguish parallel, while edge-sections of 

 kaolinite give such very decided angles that no confusion of tbe two minerals appears 

 possible. 1 have used for these comparisons the well-known kaolinite crystals from 

 Anglesey, and also those which occur abundantly (though of smaller size) in tbe 

 iuterstices of many coarse sandstones and grits of the Coal-measures. 



