268 W. M. Hutching s — The Origin of some Slates. 



rhombs ; sometimes of six-sided figures by the addition of two very 

 small faces truncating the acute angles of the rhombs. These 

 forms correspond with tbe clino-pinacoidal section of epidote 

 crystals, with which also the extinctions fully agree. Many of 

 these aggregates of small thin plates, all parallel to the cleavage 

 of the mica, often more or less overlapping one another, present a 

 peculiar characteristic appearance I have not seen in epidote under 

 other conditions. 



Thiirach (op. cit.) mentions a mineral as occurring among the 

 debris of several of the rocks so exhaustively examined by him, 

 which he describes as " small, light-yellow, feebly pleoohroic, 

 strongly bi-refractive crystals or rounded grains of rhombic or 

 hexagonal form, mostly tabular and clear, often showing cracks, 

 with a perfect cleavage parallel to the base." He quotes Klemm as 

 having looked on these as sphene ; says he himself also considered 

 them to be that mineral, but now looks on them as " potash- mica," 

 though, as he admits, they would not be readily recognized as such. 

 I think there is not much doubt that what he mentions is epidote 

 in the thin superposed tablets, resulting from biotite as just 

 described. 



Here and there, but very rarely, a few crystals of rutile are seen 

 in the altering mica. I am of opinion that rutile in granules is also 

 formed with the granular epidote, though it is not possible to make 

 sure of it. I have seen in some Cornish and other altered sedimentary 

 rocks just such clusters of rounded grains of rutile, with short and 

 comparatively blunt crystals of that mineral under conditions 

 which lead me to the belief that under the influence of dynamic 

 metamorphism the rounded grains are capable of taking definite 

 crystal form. 



The bleaching which accompanies this development of epidote in 

 the biotite is attended also by a rapid loss of the optic properties of 

 the mineral, — its strong bi-refraction and its figure in convergent 

 polarized light. Much of it, in large flakes, becomes so colourless as 

 easily to be taken for muscovite when lying flat, till tested optically. 



The substance of the bleached biotite seems to waste away and be 

 entirely removed from its original position. Flakes may be seen in 

 all stages of this removal, getting more and more indistinct, the 

 clusters of grains and plates of epidote remaining until finally we 

 see, throughout a section, many of such clusters without any of the 

 original surrounding mica. 



As regards the structure and arrangement of this soft shale as a 

 whole, the quartz and felspar grains are pretty uniformly dispersed 

 and the original mica lies mostly flat in the plane of bedding and 

 lamination, with, however, a sufficiently large proportion lying- 

 inclined, to justify the conclusion that the deposit took place in very 

 quiet water and was not appreciably levelled and sorted by currents. 

 There is a good deal of fragmentary organic matter present through- 

 out, and a good deal of limonite, diffused and in patches, which 

 doubtless to some extent represents a part of the iron originally in 

 combination in the biotite. 



