72 IF. M. Rutchings— Rocks of Great Whin Sill 



We will now pass on to the altered argillaceous limestones, 

 including under that head all such rocks as are still safely to 

 he recognized, microscopically and chemically, as having been 

 dominantly limestones, but which have contained sufficient shaly 

 material to provide a noticeable amount of silica and alumina, with 

 some alkali and magnesia, which latter will also be present, more 

 or less, in any case, in most of the limestones. Rocks of this class, 

 of varying degrees of admixture, occur at many points along the 

 Whin Sill, and have given rise to very interesting contact-products. 

 The new minerals formed are garnet, augite, idocrase, wollastonite, 

 epidote, hornblende, felspar, chlorite, sphene. The garnet is the 

 most persistent, being seen in all the slides examined from rocks of 

 this class, whereas most of the above minerals may be present in 

 some cases and absent in others. 



One or two typical examples will serve to give a general idea 

 of the nature of the alterations produced. Thus, from specimens 

 of not very impure limestones from Burtreeford, sections have been 

 cut which show the actual contact-line. First comes a narrow band 

 (about -^0 inch) which seems to mark some sort of interfusion. 

 Though now much obscured by fine-grained secondary calcite, it is 

 distinctly defined both on the side towards the Whin and on that 

 towards the limestone. The Whin is very fine-grained, but contains 

 a good many perfectly fresh and distinct crystals of felspar of 

 larger size. Occasionally one of these crystals projects just into the 

 edge of the interfusion-band, and may be seen to have been melted 

 away in it and left with a rounded end. 



Along the edge of the band towards the limestone lie many small 

 bnt good crystals, and some irregular grains, of idocrase. The 

 largest crystal in these particular slides is a prism -gV inch long by 

 1^0 inch wide, very fresh and perfect. A very few lie also a little 

 further in, but none occur at any distance from the contact-line. 

 It is interesting to note the mode of occurrence of these crystals, 

 some of which are completely bedded in quartz, some again ia 

 calcite, and others in grains of calcite which are surrounded by 

 quartz. 



Then comes another narrow zone, about -^^ inch, which consists 

 largely of recrystallized quartz as a sort of ill-defined mosaic, 

 intermixed with varying amounts of calcite. This quartz is all full 

 of inclusions of small garnet grains and indetei'minable microlites, 

 and clearly dates from the original metamorphism of the limestone 

 by the intruded Whin, being strongly distinguished from later 

 quartz which has filled in small cracks, etc., and which contains no 

 such enclosures. This quartz band passes abruptly into coarse- 

 grained, highly crystalline, saocharoid limestone, the calcite crystals 

 containing numerous small garnets in rounded grains. A deep- 

 coloured, very dichroic sphene, in good- sized crystals and grains, is 

 also present. 



Rather more impure limestones are represented hy specimens 

 from Rumbling Churn, near Dunstanbnrgh. Garnet is again very 

 abundant, mainly in very small crystals and rounded grains 



