202 i)E. C. E. TILLEY OiST THE PETEOLOGT OE [vol. ixxix. 



mica and the presence of albite among the quartz-grains are 

 characteristic features of both groups of rocks. The Anglesey 

 rocks have been regarded as sediments containing an admixture of 

 volcanic dust. In a less degree the green mica-schists of the 

 New Harbour Group present comparable features ; but a higher 

 grade of metamorphism is indicated by the abundant green 

 biotite. 



The true Green Schists of the Start area and the basic schists of 

 the Gwna Group {op. cit. p. 77) are obviously rocks of similar 

 origin, and both types (the chlorite -epidote and the hornblende- 

 epidote-albite-scbists) are represented. The analyses of these 

 basic Gwna Schists agree very closely with those of the Start area. 



It is, however, in the abundance of quartz- and quartz-albite- 

 veins in the green mica-schists, the Gwna green schists, and the 

 mica-schists of the Penmynydd zone of metamorphism, that the 

 closest relation with similar veins in the Start Group is found, and 

 their ascription to segregation processes during metamorphism is 

 in accord with the evidence obtained from the Start area. 



(iv) The Green Schists of Western Norway. 



The investigations of Norwegian geologists (notably Hans 

 Reusch, C. F. Kolderup, and V. M. Golclschmidt) have shown 

 that a great group of Green Schists forms a zone in Western 

 Norwajr, extending from the vicinity of Stavenes (61° 30' lat. N.) 

 southwards to the Stavanger region (59°), in which area their 

 development has been investigated by Goldschmidt. These rocks, 

 with underlying phyllites and mica-schists, are of Ordovician and 

 Silurian age, and have participated in the Caledonian movement. 

 The Green Schists are for the greater part hornblendic t}q:>es, and 

 are entirely comparable with the hornblende-epidote-albite-schists 

 of the Start area. 1 They include effusive and tufaceous types, and 

 sill-intrusions in the lower phyllites. In the latter the existence 

 of definite zones of metamorphisin has been established, and, as 

 bearing on the Start Schists, it may be noted that the normal 

 order of (1) biotite, (2) garnet, is reversed. Analyses of these 

 garnets very clearly show that they are not the normal almandine, 

 but one rich in spessartine. Furthermore, in the impurer types of 

 sediments the appearance of hornblende is again delayed. Gold- 

 schmidt notes that hornblende develops in the zone of biotite- 

 garnet-schists, but only in the vicinity of intrusives. This sequence 

 of events is clearly in conformity with that revealed in the 

 composite rocks of the Start area. 



1 [Since writing- the above, I have had an opportunity of studying these 

 rocks in the field, under the guidance of Prof. V. M. Goldschmidt. In 

 positions remote from the area of maximum metamorphism, as on Bu Island 

 (Buoen), of the Haastein Group, the Green Schists exhibit features identical 

 with those of the Start Group, and hand-specimens of the Norwegian rocks 

 are indistinguishable from the Start types.] 



