430 Transactions. 



of the depression, under marine covering strata, while on its surface are a 

 number of remnants of cover, the largest of which are composed mainly 

 of volcanic rock. Some of these which now form conical hills on the sea- 

 ward part of the sloping plateau are probably necks. 



The Shag Kiver does not follow the fault-angle closely, but is in places 

 superposed on the sloping plateau of the south-western side at a distance 

 of several miles from the axis of the depression. 



As the fault-scarp of the Kakanui - Horse Range is followed inland 

 (north-westward), remnants of the covering strata are found at its imme- 

 diate base inclined towards the scarp at the same angle as the stripped 

 floor farther back. Some of these remnants, moreover, may be seen to 

 rest upon a floor of schist (of which rock the whole of the sloping plateau 

 of the south-western side is also composed), but the rock of the fault-scarp 

 at the foot of which they lie is greywacke. Thus we have here not only 

 a clear demonstration of the tectonic nature of the depression, but also 

 an example of the general relation between the presumably older schist 

 and the presumably younger greywacke along the border of the northern 

 highland of Ota go — namely, that the older rock is on the downthrow side 

 of the fault-junction, indicating a reversal of the sense of movement, or, 

 rather, that the youngest faults, where they have followed the lines of older 

 dislocations, have done so merely because here lay lines of crustal weak- 

 ness, and not because the latest movements were a continuation of those 

 of a former period. 



The excavation of the overmass in the lowest portion of the depression 

 has followed a regional uplift much later than the movements by which 

 the depression was formed, and the effects of this are here important owing 

 to the fact that the course of the Shag River to the sea is a short one. 

 Composite topography, though traceable, is not sufficiently prominent to 

 vitiate the general account of the relief given above, which has, for sim- 

 plicity, been stated as though the whole feature had been developed in a 

 single cycle. 



For the explanation of one curious detail of the mature fault-scarp of 

 the Horse Range, forming the north-eastern side of the depression, the two- 

 cycle origin must be borne in mind. One of the streams of the fault-scarp 

 is exceptionally large, heading far back in the range, and this stream, 

 instead of debouching cleanly into the fault-angle, turns sharply to the 

 left while yet within the range-front, and flows for a mile or two parallel 

 with the scarp and separated from the fault-angle depression by a long 

 narrow spur of the undermass. It would seem that this stream when 

 it entered the depression at a higher level had built forward a large fan 

 at the base of the already mature fault-scarp, and that when rejuvenation 

 occurred it had been flowing down that radius of the fan which lay closest 

 to the scarp, so that when it cut downward it became superposed on the 

 undermass of the range-front.* 



The Oamaru District. 



The fault-scarp which forms the north-eastern side of the Shag Valley 

 dwindles in height towards the south-east. The block which it bounds 

 (Kakanui - Horse Range block) is strongly tilted to the east, and its surface 

 on that side is now a fossil plain sloping eastward and north-eastward 

 towards the sea, and dipping under and forming the floor of the marine 

 Tertiary rocks of the Oamaru district. 



* Loc. cit. (1917), see fig. 9. 



