178 DR. C. E. TILLEY ON THE PETROLOGY OF [vol. lxxix, 



Ussher's interpretation of the synclinal structure in the Green 

 ;Schists of this area appears to be dependent on the section which 

 he figures in the frontispiece of his memoir : namely, the section 

 ■between Outer and Inner Hope. This, however, cannot be used 

 to support the theory of synclinal structure in the main mass of 

 the Green Schists which lie south of the section. I am of 

 opinion that the features displayed in this section are more 

 probably explained by representing the major band of the Green 

 Schists extending from Malborough to Bolt Tail as constituting 

 the core of the anticlinorium formed by the coalescence of the two 

 bands of Green Schist, which (in the Bolt Tail area) have been over- 

 turned southwards, giving the predominant northward dip seen in 

 the section from Greystone Ledge to Bolt Tail. 



In the sections exposed between Bolt Tail and Whitechurch, 

 especially near Red Rot Cove, there are several thrusts which can 

 be seen from points near the water's edge. These thrusts have 

 a northward inclination, and are probably connected with the over- 

 folding of the Green Schists. 



On this interpretation, the mica-schists lying north of the 

 Malborough zone represent the upper mica-schists which abut 

 against Devonian rocks on the north. As Ussher noted, the Green 

 Schists of the northern band give some indication of dying out in 

 a north-westerly direction, and it is probable that the interdigita- 

 tion of mica-schist represented the incoming of those more normal 

 sedimentary conditions which have given rise to the upper beds of 

 mica-schist that are strati graphically on the same horizon as the 

 Bolt mica-schists. 



The structure of the Start District ma}% then, be shortly stated 

 as an anticlinorium with an axis pitching westwards; the 

 Green Schists form one major horizon resting on a lower group of 

 mica-schists — those of Portlemouth and Start — , and overlain by 

 the mass of mica-schists of Bolt Head : the latter being repre- 

 sented in the mica-schists lying north of the single band at 

 Malborough. From Malborough to Bolt Tail the summit of the 

 anticlinorium is traversed in Green Schists, the beds being closely 

 compressed, and forming an overfolded sequence in the coast- 

 section at Bolt Tail. 



III. The Mica- and Quartz-Mica-Schists. 

 (a) Mica-Schists of the Start-Portlemouth Area. 



These rocks are the oldest sediments recognized in the area. 

 They form a very uniform group, so constant in mineralogical and 

 petrographical character that no definite band can be singled out 

 as a datum-line for recognition of structure within the group. 

 Schistosity is usually well developed, and intricate folding and 

 gnarling on a minute scale is often seen. 



While there are patches free from quartz-veining, the greater 

 part of the area of these rocks exposed shows that the beds have 



