| Abstract/Notes | After this substance has been well observed in good occurrences, it is always recognized at once, even as isolated small patches in a slide. It has a very characteristic appearance among the other constituents. It is marked off, for instance, by its special refraction from anything else that occurs in these rocks, and has other characteristics not to be missed when once observed, but not very easy to exactly describe. As soon as it ceases to be quite inert in polarized light, as soon as the speckly polarization sets in, it has also a more or less granular appearance in ordinary light with lowered condenser, which is very characteristic.indifferent slides, and often in one and the same slide, various stages of development may be observed in it from the first speckled appearance in polarized light up to a point where flakelets of white mica are recognized as forminginit, lying “criss-cross”inall directions. It passes in this way in some cases almost wholly into mica, with apparently other substances which cannot be identified with certainty, the transition, and connection with the original substance, being all the time distinct and beyond mistake.In the inner zones, the “hornfels-zones,“ this substance is usually present in smaller amount, as already stated; but examples of its abundant occurrence are not wanting, as for instance in a hornfels from Spitzenberg, in the Harz. There is a mosaic of quartz and felspar, with brown mica and some white mica. The yellow substance lies in amongst, and surrounds, the minerals of this mosaic. It has no definite form of its own, simply occupying spaces and taking the outlines of the other minerals. It is all either in the minutely-speckly stage or variously developed from this upwards to mica; and the impression made is that if it had all developed as far as some portions of it have done, the whole mosaic-grains of the slides would be mixed in with and surrounded by white mica, as may not infrequently be observed in these contact-rocks. |
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