The Devonian Metamorphic Rocks. 35 



-opaque grains, apparently pseudomorphs after pyrite, and tourmaline 

 as an accessory (p. 58). Further on the surface stones afford 

 evidence of partially peroxidized mica-schists and quartz-schists. 



The author considers the Hope junction-section to be the most 

 complete in the area, and the beach reefs supply the evidence which 

 would otherwise be lacking in the intervals between the cliff sections. 

 Yet, for all this, the actual line of junction seems doubtful, and 

 we are left very much in the dark after studying the diagram (fig. 25), 

 where the author might easily have given us his view as to the 

 actual position of the boundary. He states in the test (p. 62) 

 that the Hope section exhibits the east coast and Kingsbridge 

 estuary 'brown rocks' in plicated conjunction with rocks ascribed 

 to the Devonian slates on the north side of Hope Headland and 

 with mica-schists on the south side of it. The crux appears to be 

 in Hope Headland itself, and the line of junction is most likely 

 to occur in that cliff rather than at the commencement of Mouthwell 

 beach, as shown in the old Geological Survey map and also in the 

 new one. Undoubtedly the presence of ' brown rocks ' in this 

 area without a corresponding development of the green schists 

 constitutes a difficulty which requires to be explained. 



Mr. Ussher admits that the line taken as a junction does not 

 correspond to an abrupt change in the character of the metamorphism, 

 but owing to the occurrence of the 'brown rocks' north of Hope 

 Headland, as well as in the Headland itself, he is content to place 

 the junction in the position assigned to it in the old Geological 

 Survey map. Professor Bonney and Miss Raisin place the line 

 further south, the former distinguishing the rocks immediately 

 south of Hope Headland as " true schists " from those on the 

 north side as " never more than schistose." Thus it would appear, 

 notwithstanding Mr. Ussher's statement that the Hope junction- 

 section is the most complete in the area, there seems yet to be an 

 element of doubt as to where the boundary-line should be drawn, if 

 indeed it is possible to draw an exact line under such conditions as 

 are presented by the juxtaposition of two opposing rock systems. 



Conclusion. 



The above details with regard to the boundary between the 

 Devonian and Metamorphic rocks are undoubtedly difficult to follow 

 by those who possess little or no acquaintance with the localities, 

 though possibly a general idea of this peculiar apposition of the two 

 series may be obtained. In the new map the boundary-line is 

 drawn with precision, but no fault is indicated, and everyone is at 

 liberty to draw his own conclusions as to the true nature of the 

 junction. It is perfectly clear that an immense change is accom- 

 plished within a short distance on either side of the boundary, the 

 intervening space for the length of a few chains being occupied 

 by rocks which sometimes partake of one character and sometimes 

 of the other. These conditions fully justify the Director, in his 

 preface, in stating that " a striking feature of the area in question 

 is the contrast i)etween the intensely folded but comparatively 



