188 DR. C. E. TILLEY ON THE PETROLOGY OE [vol. lxxix, 



commonly less than 2 inches across. Epidote forms the major 

 constituent of all these knots, and with it are associated chlorite, 

 hornblende, calcite, and albite. 



Some of the knots are made up essentially of epidote and horn- 

 blende. The knots examined from Hamstone Cove consist of 

 finely granular epidote, intergrown with albite, and accessorily 

 chlorite, hornblende, calcite, muscovite, and quartz. Some of the 

 hornblende prisms are in process of being replaced by biotite- 

 flakes. A few of the knots of Limebury Point are essentially 

 made up of ferriferous epidote. The grains of epidote are variable 

 in size, even within the one nodule, the periphery often being more 

 coarsely granular than the interior. 



No zonal arrangement of minerals can be made out, however, in 

 any of these nodules. The distribution of epidote in the Green- 

 Schist masses of the Start shows no analogy with the remarkable 

 associations of this mineral in the Landewednack hornblende- 

 schists of the Lizard. In the latter locality it would appear that 

 the original igneous rocks had suffered much decomposition by 

 weathering, with migration of material, giving rise to calcite and 

 other products, which on metamorphism have yielded banded types 

 of hornblende-epidote-schist. 



Such types are wholly absent from the Green Schists of the 

 Start area. It appears more probable that the nodules of these 

 Green Schists represent amygdales in an original volcanic rock, 

 which in metamorphism have yielded epidote as the prime con- 

 stituent. Amygdales of this character are recognized in other 

 Green-Schist areas, and it will suffice to note that in the Lake 

 Superior region such examples are provided. In the Marquette 

 greenstone, G. H. Williams l records small amygdales filled with 

 brightly polarizing epidote, often associated with calcite. 



Nowhere in the Start area can any evidence of ellipsoidal or 

 pillow-structure be observed, and, considering the very marked 

 dynamic metamorphism to which these rocks have been subjected, 

 the absence of such structures calls for no further comment. 



V. Schists oe Composite Origin. 



Under this heading are included those rocks which betray, either 

 b} r their aspect in the field, or by their mineralogical composition, 

 their character as neither normal sediments nor true igneous rocks. 

 Such rocks are often abundantly developed in association with the 

 true Green Schists, especially at the junctions of these rocks with 

 the associated mica-schists. There is every reason to believe that 

 these rocks are tufaceous in origin, or are basic ashes. 



They form a natural link between the normal sedimentary mica- 

 schists on the one hand, and the essentially igneous derived Green 

 Schists, on the other. Such types appear to have been described 

 under the title of ' the micaceo-chloritic series ' by Miss C. A. 

 Kaisin. Apart from the light that they throw on the original 



1 Bull. U.S. Geol. Surv. No. 62 (1890) p. 174. 



