44 W. 31. Hutchings — Clays, Slates, and Contact-Mefamorphism. 



marked off from the rest of the rock in much the same manner, when 

 seen in ordinary light, there is some temptation to consider them as 

 forming part of the same processes. But I think a full consider- 

 ation of them shows the fallacy of this, and that they should be 

 considered quite separately. Nor is it at all justifiable to speak of 

 the more indefinite spots, such as I am about to describe, as 

 composed of decomposition-products of a mineral, as e.g. cordierite, 

 unless the connection and transition can be absolutely proved. 



As a more general thing the spots and knots are spoken of as 

 consisting of, or containing, "aggregations of pigment," "aggregations 

 of iron-ores," or of "dark grains," or of "carbonaceous material," 

 or they are described as consisting of the same materials as the rock 

 in which they occur with less of some particular mineral, often 

 biotite or white mica; or also as representing "less developed 

 portions of the general mass." 



I have made a careful study of the spots in the Shap rocks, and 

 of many other occurrences, in the hope of obtaining a better under- 

 standing of them. It does not seem possible to arrive at any 

 perfectly definite and general conclusion concerning them ; but 

 certain things appear to apply to many, and to allow of some slight 

 inferences as to their possible nature, so that it may be worth while 

 to record a few observations made on them. This can, perhaps, best 

 be done by describing one or two particular cases first. 



Thus a " Fruchtschiefer " from Tirpersdorf, in Saxony, consists 

 mainly of white mica, mostly lying flat in the plane of cleavage, 

 but also a good deal lying edgewise in various degrees. There is 

 a little quartz-mosaic, a good deal of biotite, some of it in large 

 individuals, lots of small tourmalines, zircons, etc. The whole is 

 rich in dark grains and plates, many of which are transparent or 

 translucent ilmenite. The numerous spots are about the size and 

 shape of small grains of wheat, and do not seem to have any definite 

 relationship to the rock-cleavage. Under low power, in ordinary 

 light, they are sharply marked off from the surrounding material 

 by being very much richer in dark gi-ains and plates, and by having 

 yellow colour. The boundary of this yellow colour is mostly quite 

 sharp. Biotite occurs in the spots as small flakes, and larger 

 individuals of it sometimes lie partly in them and partly outside. 



In polarized light, again, the spots are marked off" very sharply, 

 being much darker than their surroundings, and containing much 

 less vividly-polarizing mica, at the same time that the contrast is 

 heightened by an extra accumulation of such mica as a frame 

 immediately round them. At a thicker, central portion of the slide 

 (which, however, must be a really thin one to make out anything 

 distinctly) the sjDot is seen to contain a great deal of a mosaic of 

 small grains or flakes, or both, which cannot be all identified, but 

 some of which is mica and some probably quartz. It can be seen 

 that these grains, etc., lie in, and are quite distinct from, the yellow 

 substance which colours the spot and surrounds all its contents like 

 a sort of groundmass. In nearly all these thicker spots one gets 

 the impression that there is, among the other constituents, more or 



