The Devonian Metamorphic Rocks. 33 



Nature of (lie Boundary between the Northern and Southern Areas. 



This is perhaps the most interesting chapter in Mr. Ussher's 

 memoir. "On the whole," he says, "the boundary of the Meta- 

 morphic area can be located with tolerable certainty owing to the 

 comparatively unaltered condition of the slates and shales on the 

 north side. The rapid change from one group to the other is 

 equally evident where the green schist series is in contact with the 

 slates, and between Malborough and Hope, where the mica-schists 

 are in contact with them." This is a terrible blow to the old notion 

 of progressive metamorphism, and is all the more severe because it 

 includes in the Devonian or northern area rocks which were mapped 

 as Metamorphic in the old Survey sheets. Professor Bonney and. 

 Miss Eaisin have done good service here, and although Mr. Somervail 

 was able to effect a slight correction in the latter author's map, it 

 only served to shift the boundary-line somewhat to the northwards. 



When we come to the consideration of local details, it must be 

 admitted that there is a certain degree of obscurity as to the actual 

 position and character of the boundary on the east coast (Green- 

 straight). This has struck more than one observer. Mr. Ussher 

 says that " there is no break in this section, which could be taken as 

 a fault boundary, or satisfactory junction between altered and un- 

 altered rocks." This seeming obscurity on the east coast, which is 

 usually the first point visited, may in part account for the contro- 

 versial character of the entire subject as to the relations of the 

 Devonians to the Metamorphics. 



There seems to be better evidence further inland, for at Kellaton, 

 he says, unaltered grey slates of the Devonian group become 

 gradually intercalated with "yellowish-brown beds and harder 

 rotten brown stony bands, giving place to soft peroxidated and 

 yellow-brown materials with seams of soft reddish and dark-grey 

 shale or schist with highly micaceous surfaces and whitish inter- 

 filming. The section appears to be a more or less rapid passage 

 from unaltered sedimentary to decomposed volcanic material, which 

 cannot be separated from the green schists." Since these 'brown 

 beds ' appear in the junction sections to be more or less connected 

 with the green schists, it is only natural to suppose that they con- 

 stitute a sort of decomposed and peroxidized selvage of those rocks, 

 into which most probably portions of the Devonian slates may have 

 been forced by pressure. They are in no sense passage beds, but 

 simply portions of the green schists in a state of decay owing to 

 their peculiar position. Mr. Ussher had previously stated (Proc. 

 Geol. Assoc, vol. xvii, p. 123) " that the junction between the 

 Devonian and altered rocks is almost invariably marked by the 

 presence of rusty-brown decomposed rocks, generally containing 

 hard carbonates. These rocks often intervene for a distance of 

 several chains. The contrast between the comparatively unaltered 

 Devonian slates and the highly altered rocks, on either side of this 

 neutral zone, is everywhere strongly marked ; nor were any 



DECADE v. — VOL. II. — NO. I. 3 



