196 DR. C. E. TILLET ON THE PETROLOGY OF [vol. lxxix,. 



regard, them as an altered series of basic igneous rocks allied to the 

 diabases in composition, and possibly consisting in part of altered 

 tuffs. It is in every way probable that the true Green Schists 

 represent contemporaneous lava-Hows, possibly associated with sills 

 of dolerite. Sediments are not scarce among them, but the great 

 thickness of bands of uniform composition, consisting solely of 

 material of igneous origin, their persistence along the strike, and 

 the undoubted tufaceous character of beds associated with them 

 are all in favour of this conclusion. The epidotic knots — con- 

 sisting of epidote, chlorite, calcite, and albite — which are found at 

 various points throughout the group are most satisfactorily inter- 

 preted as metamorphosed amygdales. 



The rocks that have been grouped as composite rocks have the- 

 undoubted character of basic ashes or tuffs. Abundant quartz and 

 white mica show their relationship to the mica-schists, while they 

 are linked to the true Green Schists in the abundance of chlorite, 

 albite, and epidote. They show a variability of composition highly 

 characteristic of rocks of this class, for they pass out into types- 

 practically free from igneous material, assuming the character of 

 the normal mica-schists. These relations can be studied, both in 

 the field and within the limits of a microscope-slide. 



These rocks are found in all parts of the Start area, both at the 

 base and at the top of the thick bands of Green Schists, and also 

 intercalated within them. 



The metamorphic features of the Start Schists are among the 

 most characteristic of those of low grades of djmamic meta- 

 morphism, and the resultant rocks find a place in the ' epi-zone ' 

 of TJ. GrubenmaniVs classification. The mica- and quartz-mica- 

 schists correspond in all essentials to the group of rocks classified- 

 by Grubenmann in his ' upper zone ' as sericite-albite-gneisses. 

 In particular, we may note the general absence of biotite in these 

 rocks, and the accompaniment of sericite by weakly coloured 

 chlorite and less abundant albite, the latter normally in intimate 

 association with quartz. 



An unusual type is the porphyroblastic albite-mica-schist from 

 Bolt Head, in which the porphyroblasts of albite are clouded with 

 carbonaceous inclusions often zonally arranged. There is clear 

 evidence that these porphyroblasts have developed in situ, in- 

 cluding swarms of carbonaceous particles during growth. 



The higher grade of metamorphism represented by the presence 

 of biotite, metamorphically developed from sericite and chlorite, 

 as I have already noted, is never reached. 1 



Among the true Green Schists two distinct types have been 

 recognized, the one characterized by chlorite, and the other hj 

 hornblende. 



Chlorite-epidote-albite-schists are the most characteristic low- 

 grade dynamically-metamorphosed equivalents of basic igneous 



1 Exception can here be made for the beginning of biotite formation seen 

 only in a few slides of mica-schists. This incipient development is found in 

 the immediate vicinity of grains of iron-ore. 



