570 MISS M. I. GAEDINER ON CONTACT- A.LTERATION 



Prof. Bonney and Mr. Alport in describing the similar altered grits 

 from the Bennan Hill to the south of New Galloway *. This tint 

 is due to the presence of brown mica in continuous planes or 

 in thick-set, rounded patches. Amongst the grits are bands of 

 pyritous shale, and one band, 6 or 8 feet thick, of spotted shale. 

 Above the rickety little wooden bridge is a thicker band of shale, 

 which I take to be that seen farther up the hill, and which, at its 

 contact with the granite, is entirely recrystallized. Above the shale 

 lie grits, like those below, and then follow more shales with highly 

 altered schistose bands, very silvery, and containing minute garnets. 

 To these succeed beds like these highly altered bands, but rather 

 more quartzose. These I suppose to be altered flags. They are 

 not well exposed in the stream, but there is a good exposure a few 

 yards from the right bank. 



A microscopical section of the purple-brown grit (PL XXIII. 

 fig. 6.) shows large quartz-grains with here and there a felspar, and 

 highly dichroic brown mica, in flakes about '01 inch long. Amongst 

 the purple-brown grits are bands and patches, from an inch to a foot 

 or two broad, containing no mica and much more clayey matter. 



The knotted schist is different from that of Skiddaw. It is not so 

 compact, and the knots are larger, being from § to ^ inch in diameter. 

 The microscope shows the Skiddaw knots to consist of one or two 

 crystals of a mineral containing many small inclusions. A few of the 

 knots in the Knocknairling-Hill rock have the same structure, but the 

 inclusions are more numerous. As a rule the knots are only formed 

 of material much finer in grain than the bulk of the rock. They 

 are sometimes sigmoidal in form, and their outlines are continuous 

 with the wavy folds into which the rock has been thrown. 



The flaggy beds mentioned as containing garnets show in a micro- 

 scopical section more brown mica than the grit, though they are 

 grey in colour. From side to side of the slide sweep wavy bands 

 of minute flakes, polarizing in bright pink and green, probably 

 sericite. The small garnets, about '015 inch across, are colourless 

 in the section, and fairly well formed. A high power shows them 

 to contain inclusions, probably of quartz, whose rounded outlines 

 Avith bays and gulfs remind one of the outlines of gas- and fluid- 

 inclusions in quartz. 



§ 3. Shales traced along their Strike to their JuisrcTioisr 

 WITH THE Granite. 



The Shales B of the map (fig. 1) are frequently exposed for about 

 ^ of a mile from the granite margin. They are dark shattery 

 shales, a good deal iron-stained and often crumpled into frills and 

 zigzags (PI. XXIII. fig. 4). The plications have frequently parted, 

 and the interspaces been filled with wavy bands and lenticles of 

 quartz, from an inch or two broad down to microscopic dimensions. 



* " Report on the effects of Contact-metamorphism exhibited by the Silurian 

 Rocks near the town of New Galloway," Proc. Royal Soc. toL xlvi., 1889, p. 200. 



