part 4] PETKOLOGi' OF THE AltNAGE DISTRICT. 449 



equal to that of the andalusite-schists ; but, since beds of anda- 

 lusite-schist occur throughout this eastern sub-group and beds of 

 quartzite throughout the western andalusite-schist, it is sufficient 

 for the present purpose to consider these two sub-groups as forming 

 one — that of the Fyvie Series, although it is possible that in 

 this extended Fyvie Series there may be included the equivalents 

 of more than the Boyndie Bay Group of the Banffshire coast- 

 section. 



The main rock of the Fyvie Series west of the Arnage Mass is 

 a nodular andalusite-cordierite-schist, well exposed along the 

 Ythan gorge between Methlick and Gight. It is a greyish, rather 

 micaceous rock from the weathered surfaces of which project large 

 crystals of andalusite measuring as much as 2 cm. in length. In 

 slices, the ground-mass of the rock is seen to be composed of coarse 

 quartz-grains, biotite-flakes, and rather large white micas ; in this 

 base are porphyroblastic masses of both cordierite and andalusite, 

 the former occurring as dirty greenish-yellow irregular patches, the 

 latter as large granular crystals. Both include much ground-mass 

 material, and the cordierite holds also chlorite in large laths. 

 Occasionally, small brown staurolites occur between the large 

 andalusite porphyroblasts. 



The andalusite-schist of the Boyndie Bay Group has been 

 analysed by Mr. F. G. Eadley, and, for reasons already given, this 

 analysis may be taken to represent that of the normal andalusite- 

 schist of the Fyvie Series. This analysis, and one of the Macduff 

 Slate into which both Fyvie and Boyndie Bay Groups pass, are 

 set forth in Table I, Analyses I & II, p. 450. They represent argil- 

 laceous rocks, differing from that which Prof. V. M. Goldschmidt l 

 has styled the typical clay-rock (Anal. T, Table I), chiefly 

 in the relative amounts of the alkalies. The Scottish rocks are 

 much richer in soda than is Prof. Goldschmidt's type. 



The felspathic quartzites of the eastern division show small 

 felspars and greasy quartzes in a scarce fine-grained base. In 

 slice, pebbles, often rounded, of quartz, orthoclase, and scarcer 

 oligoclase are set in a scanty ground-mass of small biotite-flakes, 

 quartz-grains, and a little dirty decomposed felspathic substance. 

 Others of these gritty rocks approach the grey wacke type ; these 

 are liner in grain, and consist of small, often angular grains of 

 quartz, oligoclase, and potash-felspar in a ground-mass of biotite, 

 muscovite, quartz, felspar, and magnetite. 



The quartzites vary from fairly pure quartz-rocks to felspathic 

 quartzites containing some 20 per cent, of felspar, the purer types 

 being more common around the Arnage Mass. The chemical 

 composition of an average quartzite concerned in contamination 

 may be taken as somewhat like that of the Cullen Quartzite 3 of 

 Banffshire, an analysis of which is given in Table I, Analysis IIT, 



1 ' Die Kontaktmetamorphose im Kristianiagebict ' Videnskap. Skrift. I, 

 Mat.-naturv. Klasse, 1911, No. 1, p. 16. 



2 H. H. Read, ' The Banffshire Coast-Section of the Highland Schists ' 

 App. I to ' Summary of Progress for 1920 ' Mem. Geol. Surv. 1921, p. 72. 



