576 



MISS M. I. GAEDINER ON CONTACT- ALTERATION 



round the edges of the light lenticles. They are often thick!}' 

 studded with small garnets. Branching masses of a somewhat 

 ivory -like material occur. A lens shows this to contain many very 

 small flakes of white mica. These are packed in a substance which, 

 when crushed, breaks into numberless small needles. 



Pig. 3. — Junction of the Granite and Altered Grits, on the 

 southern side of the Third Boss, shovjing quartz-garnet lenticles. 



sign and extinguishing parallel to their length, hence probably 

 sillimanite. The microscope shows (fig. 2, PI. XXIII.) that the rock 

 has lost all signs of its clastic origin. As seen in the upper part of 

 the slide, it consists of quartz and white mica, both containing 

 sillimanite, with brown mica and garnets. To the right is indicated 

 the cloudy look of one of the mica-sillimanite aggregates. 



The main part of the lenticles, as seen at the bottom of fig. 2, 

 PL XXIII., consists of garnets. These give to a freshly-broken 

 surface a delicate pink tint. They are set in clear vein-quartz, 

 often in large pieces, having an angular outline, and traversed by 

 lines of liquid- or gas-inclusions. The sillimanite-raica aggregate 

 just described occurs in large branching masses, often an inch or 

 two long. 



I have had no opportunity of comparing the less altered grits 

 with the " quartzite micace " of Guemene, which, from the de- 

 scriptions, they seem to resemble ; but I have seen a specimen and 

 slide of the " leptynolithe granatifere " of Eostrenen. The " lepty- 

 nolithe " is a compact dark jjurple rock, looking microscopically less 

 altered than the Xew Galloway rocks ; but the microscope shows it to 

 be as entirely recrystallized, with these differences, — that it is much 

 finer in grain, and that the minerals are not so well formed. I sent 

 a specimen of the most highly altered grit to M. Barrois, and asked 

 if he would kindly tell me whether it resembled the " quartzite 

 sillimanitise " of Guemene ; he answered that there was no such 

 rock amongst the Silurians or Devonians in France, but only 

 amongst rocks of doubtful origin. He further says that the spe- 

 cimen is identical with the " Archeans '' of Xantcs metamorphosed 

 by granulite. 



