W. M. Hutchings — Clays, Slates, and Contact-MetamorpMsm. 65 



appearance in ordinary light with lowered condenser, which is very 

 characteristic. In different 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 forming in it, lying 

 " criss-cross " in all 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. 



Before discussing the probable nature of the substance above 

 described, I will recur to the spots in the contact-slates at Shap. 

 These spots are all marked off sharply by being relatively free from 

 biotite in a rock which is very rich in that mineral, so that they 

 appear by comparison in ordinary light as clear spaces, varying from 

 almost coloui'less to a good deep yellow in some cases. The aggre- 

 gations in them of anatase, ilmenite, etc., have been formerly 

 described by Harker and Marr and myself (Quart. Journ. Geol. iSoc. 

 vol. xlvii. 1891 ; and Geol. Mag. October, 1891). 



Examination in polarized light shows them to be of several kinds, 

 and when a sufficient number of specimens are examined it does not 

 appear possible to point out any special development in their nature 

 as taking place steadily as we approach the granite. Indeed, it may 

 be remarked of the contact phenomena in general that they are 

 somewhat erratic as we approach the contact up Wasdale Beck, 

 which may probably be due to irregular contour of the granite below 

 the surface. 



Harker and Marr look on these spots as being "evidently im- 

 perfect crystals, charged with a large quantity of foreign inclusions," 

 pointing out that some of the spots are sufficiently individualized to 

 extinguish, as to their main constituent, pretty uniformly over the 

 whole area and frequently parallel to the longer axis. They think 

 the mineral present is likely to be andalusite, and I was formerly 

 of the same opinion, but after renewed examination of the slides I 

 am no longer so. Further study of the spots of this nature, and 

 careful comparison of them with some continental occurrences, 

 show that cordierite is the mineral in question. It is tolerably 



DECADE IV. VOL. I. — NO. II. 5 



