part 4] petrology or the arnage district. 451 



for the purposes of this communication they may conveniently he 

 styled the Ellon Series. 1 Two main rock-types make up this 

 series: (1) dominant biotite-gneisses, schists, and granulites, and 

 (2) subordinate hornblende-schists. 



The first type is seen, among other localities, in the southern 

 bank of the Ythan at Esslemont House, at Craigs of Auchterellon 

 near Ellon Station, at Craighall, and in the Braes o' Waterton, 

 1 mile east of Ellon. The strike of the foliation-planes along the 

 Ythan is north-east to south-west, or north-north-east to south- 

 south-west ; dips are variable, and mainly low, on both sides of 

 these lines. 



The rock-varieties of this type are all greyish granulitic 

 schistose or gneissic rocks composed mainly of quartz, felspar, and 

 biotite ; garnet and cordierite are rare. The granulitic variety 

 shows in slice quartz-grains as the dominant component, together 

 with a considerable amount of unoriented biotite (pleochroic from 

 pale yellow to deep brown) and a fair quantity of felspar ; clouded 

 orthoclase is the chief species of felspar, less common are micro- 

 cline and oligoclase ; zircon is accessory: the rock is a quartz- 

 felspar - granulite. In the gneissose varieties the same 

 minerals occur, but here the dominant felspar is oligoclase, which 

 is, in some specimens, almost as abundant as the quartz ; garnet 

 in scarce small crystals is sometimes seen in these rocks. The 

 third main variety is a sbaly biotite-schist, with abundant 

 orientated biotite and rarer muscovite, separated by bands of 

 granulitic quartz with scarce plagioclase-grains. From the Braes o' 

 Waterton comes a cordierite-biotite-schist, in Avhich, in 

 addition to the usual minerals of these rocks, occur streaks of 

 cordierite now replaced b}" micaceous decomposition-products. All 

 the sedimentary rock-types of the Ellon Series are streaked with 

 quartz-lenticles and stringers. 



The three main types are found in intimate varying association 

 over the ground south-east of the igneous and contaminated rocks 

 (see fig. "1, p. 452). In its mineral-content the Ellon biotite- 

 gneiss is very like the oligoclase-biotite-gneiss of the Cowhythe 

 Group 3 of the Banffshire coast, and the analysis of this gneiss 

 (Analysis IV, Table I, p. 450) may be taken to represent one 

 common variety of the sedimentary types of the Ellon Series. 



The hornblendic rocks of the Ellon Series can be studied in 

 their unaltered state in the Braes o' Waterton. The rock is a well- 

 foliated dark-green hornblende- schist, which in slice is seen to 

 be made up of stout hornblende-prisms, pleochroic in shades of 

 yellow and green, with a subordinate amount of quartz and felspar. 

 The hornblende-prisms are collected into bands separated by layers 

 of quartz, felspar, and smaller and scarcer hornblende-grains. 

 The felspar is quite clear and recrystallized ; it varies from 



1 Sec J. S. Grant Wilson, ' Explanation of SLeet S7 ' Mom. Geol. Surv. 

 Scot. 1886, pp. 7-9. 



- H. IF. Read, App. I to ' Summary of Progress for 1920 ' Mem. Geol, Surv. 

 1921, p. 75. 



