386 PROF. P. MARSHALL ON THE [Aug. I906, 



In the terminology proposed by Prof. W. M. Davis, these facts 

 may be thus expressed. The North-East Valley and the Leith- 

 Kaikorai valleys are those of consequent streams, resulting from 

 the high elevation of the volcanic district. The lower part of 

 the Leith gorge is a subsequent. The valley behind Maori Hill is 

 an air-gap, and down this a more subsequent or obsequent stream 

 flows into the Leith. 



The submerged stream-valleys that now constitute the Upper and 

 Xower Harbours, Hooper's and Papanui Inlets, are sufficient proofs 

 of the fact that the district has been, within geologically-recent 

 times, at a much higher level relatively to the sea, A minimum 

 estimate of this previous elevation can be obtained by producing 

 the actual slope of the sides of Otago Harbour at its narrowest 

 point until the slopes meet at the bottom of a Y-shaped valley. 

 This would take place at a vertical depth of 900 feet below the 

 present sea-level. The inlets are all of considerable breadth near 

 their entrances, and may possibly extend outwards beyond the 

 coast-line. It is, therefore, probable that the elevation was much 

 greater than the minimum estimate given above. 



On the other hand, there are some physiographical features that 

 show that the land has been more deeply submerged than at 

 present since these valleys were eroded. In many places along the 

 coast-line old marine rock-shelves can be found. They are especially 

 noticeable near Cape Saunders, at Sandy Mount, and at Sea View. 

 The most distinct of them is about 250 feet above the present sea- 

 level. At Sandy Mount the change of sea-level has been responsible 

 for the formation of the magnificent natural bridge. The span of the 

 bridge is 250 feet above sea-level and 30 feet wide. Its surface is 

 level with the old marine plain or shelf. The ravine that it spans 

 follows the course of a tinguaite-dyke. When the land-level was 

 250 feet lower, marine erosion worked the coast-line back 100 yards 

 farther than the present sea-margin, and this old sea-margin is now 

 marked by a ridge of worn cliffs. As elevation proceeded the coast- 

 line reached farther forward, but is now being worn back again, 

 though with extreme slowness, because of the hardness of the rock. 



Along the tinguaite-dyke, planes of rock-fracture have enabled 

 the marine action to proceed with greater rapidity, and a ravine 

 with vertical walls, except where landslips have sloped it off, has 

 resulted. The erosion has been confined to the sea-level, and at 

 the sea-margin, where the diameter of the dyke increases slightly 

 upward, some of the tinguaite has been supported in place, and, 

 constituting the arch of the natural bridge, thus prevents the sides 

 of the ravine from slipping. 



There are no marine shelves or other indications in this district 

 tending to show that the submergence has ever been greater than 

 300 feet below its present level. The identity of level, between the 

 inner and outer portions of the sand-flats at the entrances to the 

 inlets, shows that the present level has been maintained for a long 

 period of years. 



