472 MB. H. H. BEAD 03" THE [vol. lxxix, 



Locality 9. — Wood, south of the Ythan, 450 yards 

 south of Glencroft. — Here crags of coarse fluxional granitic 

 end-product, with no visible xenoliths, occur. The width of the 

 outcrop in an east-and-west direction exceeds 200 yards, and on 

 both sides are passages to a cordierite-biotite mixture of Arnage 

 Type, with multitudes of blue hornfels-xenoliths. The fluxion in 

 the Ardlethen Type strikes north 5° west. 



Locality 10. — Glencroft Quarry. — The quarry itself is in 

 solid granite. Westwards and south-eastwards the granite is mixed 

 with contaminated rocks of Arnage Type bearing blue hornfels- 

 xenoliths. On the furze-clad ground north of the quarry the 

 granite is often xenolithic, and passes northwards into the time 

 Ardlethen Type, and this into the Arnage Type still farther north. 



These localities show, therefore, gradual transitions from a cor- 

 dierite-bearing Arnage Type with xenoliths to a granitic Ardlethen 

 Type without xenoliths, and in one place (G-lencroft Quarry) the 

 Ardlethen Type appears to pass into a granite of normal character 

 and aspect. A common fluxion is seen in both the Arnage and the 

 Ardlethen Types, and the latter occurs as schlieren, bands, or big 

 lenticles in the former. It will be remembered, too, that in the 

 main development of the Arnage Type, north of the Ythan around 

 Arnage, there are granitic bands and streaks of small size. 



Petrography of the Ardlethen Type. — In hand-specimens 

 the granitic rocks of Ardlethen Type always show a pronounced 

 fluxional structure, by which the large felspars (up to 1 cm. long i 

 and quartz become lenticular, and are wrapped about by biotite 

 films so that the rock simulates an augen-gneiss. In colour, the 

 rocks are pale (the felspars being whitish or pinkish), and streaked 

 with dark biotite-films. 



In slice, the components of the Ardlethen Type are seen to be 

 quartz, various felspars, and biotite, with less common garnet, 

 cordierite, and spinel. 



The quartz forms aggregates of grains often showing undu'lose 

 extinction ; the alignment of the larger axes of the aggregates 

 helps to provide the fluxional structure. The felspars are of 

 various kinds : chief is oligoclase in large crystals, often 

 showing pressure-effects, and lenticular in outline. The plagioclase 

 is rarely as basic as andesine. Less important than the plagioclase, 

 though still abundant, are orthoclase and microcline, which 

 also occur in large plates similarly lenticular. The orthoclase is 

 normal, and rarely shows microperthitic structure. The micro- 

 cline has a well-developed cross-hatching and a large optic axial 

 angle : its albite-molecule must be subordinate, in contrast to its 

 importance in the soda-microcline of certain rocks of the Arnage 

 Type already described. Grains, often of large size, of micro- 

 pegmatite are common, and often almost completely surround 

 alkali-felspar crystals ; grains of myrmekite are rare. Biotite, 

 pieochroic in pale yellow to deep brown-black, occurs either in 



