78 TF. M. EutcMngs—Eoclis of Great Whin Sill. 



liighlj' metamorphosed beds all original quartz has disappeared, and 

 Las been replaced by newly-formed contact-quartz. Where the 

 amount of it is sufficient, we sometimes get a good "mosaic" of 

 the same nature as what we see so universally at granite-contacts. 

 Where there is less of it, we see it disseminated among the micaceous 

 jiart of the rock in single grains, and groups of grains. In less 

 intensely affected cases we get more or less clastic quartz remaining; 

 sometimes it does not seem to have been attacked at all, and again, 

 we may be able to see various degrees of its attack and corrosion 

 by the processes of solution which took place. With the more or 

 less regenerated quartz we nearly always see that the argillaceous 

 position of the shale has given rise to just the same products as 

 those we have been considering, the mica and chlorite, the spots, 

 and the residual speckly substance, all appearing in the same 

 relationships as to individual forms and general structures. 



Among these altered sandy shales, however, there are some 

 occurrences which are of such special interest that they must be 

 here alluded to, in connection with the review of the whole contact- 

 phenomena of the Whin Sill and their beai'ings on the general question 

 of contact-metamorphism. In a former paper ("An Interesting 

 Contact-Eock," Geol. Mag,, March and April, 1895) I gave minute 

 descriptions of the rocks to which. I allude, and I would refer 

 students of the subject to that paper, limiting myself here to 

 a recapitulation of the particular points involved. 



The principal rock in question is a bed of shale 8 feet thick, 

 at Falcon Glints. It occurs 75 feet below the Whin, a series 

 of limestones, sandstones, and shales intervening. It contains at 

 some parts large numbers of approximately spherical nodules like 

 peas. Thin sections show, in ordinary light, a grey groundmass 

 in which are bedded grains of clastic quartz. In polarized light 

 it is seen that this groundmass consists largely of an isotropic 

 substance, in which lie numerous grains, rounded, irregular, or more 

 or less definitely-bounded, of newly-formed quartz and some felspar. 

 At parts these grains are so numerous and closely packed that they 

 amount to a true interlocking mosaic, with very little isotropic 

 matter. At other places they are more separated, and we get quite 

 large spaces of the isotropic substance, but containing small flakes 

 of mica and other things. These grains are not yet fully indi- 

 vidualized ; they are not water - clear, and have still so much 

 dimness about them that they cannot be properly made out at all 

 except in polarized light. They are absolutely distinguished from 

 the original clastic material; not one of them could ever for a 

 moment be mistaken for anything but a newly-formed secondary 

 product. 



The clastic quartz-grains remaining are seen to be all more or 

 less attacked and corroded by the surrounding groundmass ; their 

 original angular outlines are in nearly all cases preserved, but the 

 outer portions are no longer quartz, but an altered substance often 

 containing a considerable amount of white mica and sometimes of 

 felspar, whilst in some cases anthophyllite and andalusite are seen. 



