562 The Rev, Dr. Irving — Dynamic Metamorphism. 



1000 to 2000 fathoms. The deposits appear to be of Pliocene or 

 early Pleistocene age, and are covered by the raised coral-reefs. 



Here, then, we seem to have a series of changes corresponding to 

 those indicated by Prof. Spencer (loc. cit. p. 209), and it is surely 

 more probable that the movements in the two contiguous areas were 

 contemporaneous than that so great a submergence in the one 

 should correspond with a period of high elevation in the other. 



There is however an objection which Mr. TJpham might urge 

 against this view, and which I may as well anticipate : it is contained 

 in his remark that the depression " evidently took place in a 

 late geologic period, else the channels would have become filled 

 with sediments." But have they not been partially filled up? 

 Dr. Spencer remarks, " The soundings show that to within com- 

 paratively shoi't distances of their mouths, the depths of the valleys 

 below the surface of the seas sometimes did not exceed from 1200 

 to 1800 feet, but that beyond there was a great increase in depth 

 within the last few leagues." He assumes that the present slope 

 of the floors of the valleys does not materially differ from that they 

 possessed as terrestrial surfaces, but surely the peculiar conformation 

 he describes ma}' be due to the partial infilling of the valleys with 

 sediment, for a certain distance from the shore. 



Again it is possible that the surface current of the modem river 

 creates a submarine return current, and that the action of the latter 

 causes less sediment to be laid down over the valley area than over 

 the plateaux on each side. Dr. Buchanan has investigated a deep 

 depression at the mouth of the Congo, and has gone so far as to 

 suggest that the canon-like trough is entirely due to the action 

 of such a current, the mud brought down by the river being spread 

 out over tracts to the right and left, while the tract over which the 

 main current runs is kept clear by a strong reverse current of sea- 

 water. Thus he supposes the bottom of the trough to represent 

 the original outline of the continental shelf and the shore slopes 

 on each side to have been built up by deposition of material. 



Without going so far as this, it does seem possible that, if a deep- 

 cut valley was carried below the sea more rapidly than deposition 

 could fill it up, — a submarine current might be set up which would 

 prevent the accumulation of more than a certain amount of sediment 

 within it. Consequently the valley might, remain as an open trough 

 for a long period of time, and its existence could not be regarded 

 as a proof of the recent submergence of the area. 



VII. — Note on Dynamic-Metamorphism. 

 By the Rev. Dr. Irving, B.A., F.G.S. 



I HAVE elsewhere pointed out * the importance of the two factors 

 — (1) the lateral force which produces the metataxic work 

 (qua cleavage), (2) the resistance offered to the movement of the 

 constituent particles of a rock-mass in the direction of cleavage-dip 

 (in some cases) by the dead weight of the superincumbent mass ; 



1 In my " Chemical and Physical Studies in the Metamorphism of Rocks," p. 56. 



