part 2] THE METAMORPHOSED ROCKS OE THE START AREA. 191 



from the undoubted Devonian by an important dislocation. The- 

 former, in particular, is emphatic with regard to the prior state of 

 metamorphism of the Start Schists, and has classed these as 

 Archsean. 



The evidence which has been adduced for the theory of pro- 

 gressive metamorphism is of a very doiibtf ul character. Holl l 

 was of opinion that the southern area represented an area of 

 contact-metamorphism mantling a hidden mass of granite. This 

 view is wholly unwarranted. There is no evidence that thermal 

 , metamorphism has played any part in the development of the 

 schists. On the contrary, the mineralogy of these rocks is that 

 characteristic of dynamic metamorphism. 



The views of Hunt with regard to analogies in mineral com- 

 position between Devonian rocks and members of the Start Schists 

 are scarcely cogent evidence for any correlation of these two* 

 groups of sediments. 



The hypothesis of a boundary of unconformity can be dismissed,, 

 for there is no evidence by which such a view can be sustained. 



For the view that a separate boundary of dislocation separates 

 the southern schists from the undoubted Devonian sediments, there 

 is much support. The great development of Green Schists in the 

 Start area can find no parallel in any igneous horizon in the' 

 Devonian rocks of the northern area. Wherever continuous- 

 exposures are developed between Devonian slates and the Start 

 Schists, the determination of a boundary-line within a few yards is- 

 never left in doubt. Ussher, in the memoir already quoted, devoted 

 a chapter to this evidence for a boundary, and the data brought 

 forward are so convincing that there is little to add to his- 

 descriptions. 



The principal sections which afford indications of the relations 

 of the two groups of rocks are the coastal and estuary exposures, 

 those of Hall Sands, Southpool Creek (particrdary the west side), 

 the east and west sides of the Kingsbridge estuary, north of 

 Scoble and Ilbertstow Points, and lastly the section revealed at 

 Hope on the western coast. 



The Devonian slates at all these points are highly inclined, with 

 a dominant cleavage-dip northwards, and the bedding near the 

 junction, where it can be ascertained by the presence of siliceous 

 or calcareous bands, has also a strong northward inclination. 

 At the immediate junction where Green Schists are present, they 

 have been converted into ' brown rocks ', owing to a secondary 

 development of iron-oxides. This ferruginous development is 

 characteristic of the rocks along the whole length of the boundary- 

 line, and in itself is evidence favouring dislocation. 



One of the best sections for studying the relations of the rocks 

 at the boundary is that displayed on the western shore of South- 

 pool Creek, near Gullet Quarry. The evidence of a dislocated 



1 Q. J. G. S. vol. xxiv (1868) pp. 438-39, 



