part 2] THE METAMORPHOSED ROCKS OF THE START AREA. 201 



indicator-beds in areas of progressive metamorphism, and the 

 range of metamorphism expressed in the ' green beds ' of Scotland 

 provides a clearer view of the grade of alteration impressed upon 

 the Start Group. This range of metamorphism is very clearly 

 indicated in the exposures of the ' green beds ' in Central Perth- 

 shire, as at Aberfeldy and north-east of Loch Katrine. Proceeding 

 northwards, one notes that the grade of metamorphism is gradually 

 increased. 1 This can be very clearly recognized from a suite of 

 rock-sections in the collection at the Sedgwick Museum, Cambridge. 

 The order of increasing metamorphism is supplied in the following 

 rocks : — 



11935 Chlorite-albite-schist North-west of Bienacreag, Loch 



[muscovite, epidote]. Lubnaig. 



11898 Chlorite-biotite-albite-schist East of the head of Loch Lubnaig. 



[muscovite, epidote]. 

 11934 Biotite-albite- schist Eastern end of Loch Katrine. 



[epidote]. 

 6908 Garnet-biotite-albite-schist West of Loch-na-Craige, Aberfeldy. 



[chlorite]. 



11936 Garnet - biotite - hornblende- South-east of Aberfeldy. 



schist. 



This series illustrates very clearly two important points when 

 we are considering the analogous rocks of the Start area. First, 

 we ma} r note that the appearance of hornblende is delayed, as in 

 the Start Schists. It is highly probable that in the use of horn- 

 blende as a zonal mineral, the presence or absence of sericite in the 

 original sediment or rock must be considered. 



The second point illustrated is the order of appearance of garnet. 

 In these rocks it has followed biotite, thus conforming to the 

 normal order. In the metamorphism of basic igneous rocks 

 practically free from potash, the order of development would 

 appear to be chlorite — hornblende — garnet, this garnet being an 

 isomorphous mixture of grossular, pyrope, and almandine ; but, 

 where a notable content of potash has been present (as in tufaceous 

 types), the order is 



■ biotite — garnet . 



Y 



chlorite < (Al.) j hornblende, 



garnet — biotite 

 (Sp.) 



according as the garnet is predominantly an almandine, or 

 contains notable quantities of the spessartine molecule. 



(iii) The ' Mona Complex ' of Anglesey. 



Among the pre- Cambrian rocks of Anglesey, which Dr. Greenly 

 has styled the Mona Complex, 3 there are petrological types 

 which have analogues in the Start area. The Gwna green schists 

 (op. cit. p. 67) bear a close comparison with the composite rocks 

 of the Start area. The intimate mixtures of chlorite and white 



1 Mem. Geol. Surv. Scotl. (Sheet 55) 1905, pp. 13, 110. 



2 ' The Geology of Anglesey ' Mem. Geol. Surv. vol, i (1919). 



