W. M. Hutchings — Clays, Slates, and Conftacf-Metamorphism. 45 



less of isotropic matter, referable to the yellow substance, but it is 

 not possible to make quite sure. 



If other spots be examined which project out to the thinnest 

 edges of the slide, and higher powers are used, the yellow sub- 

 stance is seen to be of a pale yellowish-green colour, free from any 

 trace of dichroism, and full of an irregular netwoi'k of exceedingly 

 fine cracks or cleavages, only well seen with high powers and much 

 lowered condenser or partly closed iris-diaphragm. In polarized 

 light the grains and flakes of the contained minerals are again seen, 

 and it is here possible to make sure that in among them are isotropic 

 spaces. Some of these may be seen of such relatively large area, — 

 so many times larger than any of the contained minerals, — that all 

 idea of compensation by overlapping individuals is dispelled. One 

 can satisfy oneself that what is seen consists of the yellow substance 

 pure and simple, and that the portion examined has no action what- 

 ever on polarized light. 



In addition to the appearances above described there are spots in 

 the same slide where the yellow substance is deeper in colour, and 

 polarizes faintly in the lowest possible grey tints. In some few 

 cases this polarization becomes distinct enough to give a tolerably 

 uniform, though fibrous or flaky, extinction over a good large area of 

 the spot, the contained minerals being seen in among this with their 

 own independent extinctions as usual. Under these conditions the 

 yellow substance sometimes shows traces of dichroism. In this 

 faintly dichroic and lowly polarizing form it more resembles, in its 

 appearance in polarized light, some occurrences of serpentinous 

 material than anything else to which I can compare it. 



This same substance, varying in colour from much deeper yellow 

 to almost colourless, and varying in its dichroism and its degree of 

 bi-refraction, may be seen again in the spots of many other contact- 

 rocks. Thus, in a " Knotenthonschiefer " from Andlauthal, in the 

 Vosges, the spots again are marked off in ordinary and polarized 

 light exactly as in the above descriptions. The yellow substance 

 is present and of the same nature, but in the outer rims of the spots 

 polarized light produces in it a very faintly-speckled, minutely- 

 cryptocrystalline effect, while in the central portions it does not 

 itself polarize but contains, as usual, a mosaic of grains, some of 

 which ai'e quartz, while a few may be made out as felspar, with 

 mica. This minutely-speckled polarization is very frequent in this 

 substance in rocks from many localities, and seems to correspond to 

 the first stage of its passage from an amorphous to a more or less 

 crystalline condition. 



In some occurrences of "Knotenschiefer" this substance is limited 

 to the spots, but there are others in which smaller portions of it 

 occur more or less abundantly outside them, in among the general 

 materials of the slates, as patches or streaks of various size. And in 

 the more inner zones of contact, after the spots have ceased to 

 appear, it is still often seen, usually in smaller amount but in some 

 cases very abundantly. 



(To be continued.) 



