part 4] PETROLO&Y OF THE ARNAGE DISTRICT. 461 



a constant bedding-dip to the north-west, and the dip of the 

 fluxion in the magmatic rocks is in the same direction. On the top 

 of Elphin Hill the contaminated rock is one typical of the Arnage 

 Type ; it holds small xenoliths of blue hornfels, and is nested with 

 much quartz. The strike of the fluxion is still north-east and 

 south-west. 



(8) Hayhillock knob, half a mile south-south-west of 

 Hay hillock. — At this locality are found associated excellent 

 examples of the Arnage Type of felspar-biotite-quai'tz-cordierite- 

 rock and hornblendic rocks of the Kinharrachie Type. All these 

 rocks are patchy and streaky, with a fluxion running north-east 

 and south-west ; xenoliths of quartzite and argillaceous schists are 

 abundant ; the smaller xenoliths, chiefly of shaly material, are 

 aligned with the fluxion, which curves in treacly bands. around the 

 larger xenoliths. There are abundant nests of white quartz 

 measuring up to 2 inches in diameter. 



From the field evidence there can be little doubt that the 

 contaminators of the initial magma which provide the Arnage 

 Type are andalusite-schist and quartzite of the Fyvie Series and 

 biotite-gneisses and schists of the Ellon Series. From certain 

 evidence given in § VI it appears that the most important con- 

 taminator for this Arnage Type is an argillaceous schist (see 

 p. 474). 



Petrography. — In hand-specimens the rocks of. this type are 

 coarse-grained, greyish or bluish, and show biotite, cordierite. quartz, 

 and felspar. Large pink garnets are usually scattered through the 

 rock ; xenoliths are seen in all stages of digestion, and there can 

 be no doubt that the matrix in which they are held was a magmatic 

 rock. In a weathered condition the garnetiferous rocks of this 

 type resemble a plum-pudding in appearance, for the felspars take 

 on a rich brown colour, and serve as a suitable background for 

 black biotites and red garnets. 



The chief mineral constituents of the Arnage Type are quartz, 

 cordierite, felspars (oligoclase-andesine, soda-microcline, and rare 

 orthoclase), and biotite ; less common, although of widespread 

 occurrence, are garnet, spinel, and tourmaline (see fig. 4, p. 4(52). 

 The occurrence of small amounts of pyroxenes in this type has 

 already been noted. 



Quartz forms large grains often aggregated into patches ; lines 

 of inclusions traverse the grains at right angles to the fluxion, when 

 this is present. 



Felspar occurs in three species. Most abundant is oligo- 

 clase-andesine in fresh well-formed crystals, often included in 

 the quartz ; this plagioclase is usually about 2 to 3 mm. long, but 

 in certain rocks it builds porphyritic crystals with a maximum 

 length of 1 cm. Extinction-angles on cleavage-flakes and the 

 refractive index indicate that it has a composition of Ab 70 An 3n . 

 Its sign is negative, but in rare cases a positive sign has been noted, 



