580 MISS M. I. GARDINER ON CONTACT-ALTERATION 



Some of the mineral alteration may be due to the energy of the 

 resisted force which caused the folding ; but the main effect must, I 

 think, be acknowledged to be due to the granite. The mica of the 

 purple-brown grit is evidently a product of contact-metamorphism, 

 for there is a zone of this rock almost all round the granite. The 

 increased metamorphism of the Knocknairling-Hill rocks as the 

 granite is approached indicates the same cause. 



The variation in the amount of alteration at the same distance 

 from the margin, — the way in which the extreme alteration of the 

 grits begins in patches which increase in size until the whole is re- 

 crystallized at the margin, — -the way in which the large quartz-grains 

 of the altered rocks contain little grains which look partly dissolved, 

 — and the signs of the transference of material, suggest to my mind 

 the action of highly-heated water. 



Polding on the large scale was certainly over when the granite 

 was intruded, for both on the east and west the granite is constantly 

 seen cutting across the folds ; but, supposing the contact-effect due 

 principally to highly-heated water, which would very likely have 

 been emitted during a long period of time following the injection of 

 the granite, might not the minute folding have taken place during the 

 same time, so that dynamic- and contact-metamorphism have taken 

 place simultaneously ? 



EXPLANATION OF PLATE XXIIL 



Fig. L Most highly altered shale : X about 50. Showing white mica, brown 

 mica (shaded), colouring matter (marked black), and. quartz. The 

 quartz is shaded, and is dotted where it is full of small inclusions. 



2. Most highly altered grit : X about 50. The upper part of the slide 



represents the main mass of the rock ; the lower, part of a quartz- 

 garnet lenticle. The minerals are white mica and quartz con- 

 taining silliraanite needles, brown mica (shaded), and garnets. The 

 cloudy-looking portions on the right, both in the rock itself and 

 in the lenticle, represent the mica-sillimanite aggregates described 

 in the text (p. 570). 



3. " Graphic "' arrangement of tourmaline : X about H. 



4. Altered shale ; drawn with a hand-lens : X about 2. Showing quartz- 



veining and arrangement of colouring matter along folds. 



5. Altered shale : X about 10. Showing elongated irregular patches of 



a biaxial mineral, probably a felspar, and bands of large -granular 

 quartz. 



6. Purple-brown gi'it: X about 50. Showing quartz-grains, brown mica, 



and a few felspar-grains ; one lies S.E. of the centre. 



7. Altered grit ; drawn with a hand-lens : X about 4. Showing garnet, 



mica, and quartz, 



8. Altered grit, with secondary cleavage ; drawn with a hand-lens : X about 



2. The shaded bands forming the Vs are masses of small light-brown 

 micas, and the interspaces fine-granular quartz. Chiastolite to the 

 left. 



9. Chiastolite-mica-schist ; drawn with a hand-lens : X about 2. Chias- 



tolite, dark-brown mica, colouring mutter, ;ind garnets. A good 

 deal of quartz in the light bent band in the middle. 



