270 Transactions. 



the lower slopes of Mount Thomas, and south of the Ashley River, where 

 they form the Mairaki Downs (= Moeraki Downs of Haast). The beds form 

 a kind of frontal apron to the higher greywacke hills, such as Moimt Grey 

 and Mount Karetu ; but still they rise in places to well over 1,000 ft. above 

 sea-level. The downs country has been dissected to some extent, and on 

 the front facing south-east, consequent streams have cut deep narrow 

 channels, with high precipitous banks, whereas in the north-eastern portion 

 the tributaries forming the Northern Kowai tend to develop valleys along 

 the strike. The same is also true of the east branch of the Grey. The 

 character of the drainage points to recent and rapid uplift, perhaps in agree- 

 ment with that of which there is distinct evidence on the coast farther north 

 (McKay, 1877, p. 177 ; Hutton, 1877, p. 55 ; Speight, 1918, p. 99). On the 

 sides of the steep banks, especially those running with the dip, numerous 

 good sections are exposed ; and it will be best at this stage to give a more 

 detailed description of typical sections, preferably those illustrating the 

 relationship betw^een the underlying Tertiary beds and the overlying gravels. 

 Although the beds are typically developed in the basin of the Kowai River, 

 and I have selected the uame of that locality as the one most appropriate 

 to designate the series, yet the most instructive sections are to be seen in 

 the basin of the Grey River, and these will therefore be taken first. 



Descriptions op Typical Localities. 

 Grey River, East Branch. 



The eastern or chief branch of the Grey River rises in the country between 

 Mounts Grey and Karetu, flows south therefrom in a deep wooded gorge, and 

 then gradually turns to the south-west and follows along the north-western 

 edge of the Mount Grey Downs till it enters on the plains and joins the Okuku 

 River in the neighbourhood of White Rock Station. The first part of its 

 course has been cut in greywacke, but on leaving the higher country it crosses 

 the marginal fringe of Tertiary sedimentaries at an angle of about 45° with 

 their strike, so that when the stream runs in the direction of the dip the 

 cross-section of its channel is narrow and trench-like, but when it runs along 

 the strike the valley opens out somewhat, with dip slopes bordering the 

 stream on its north-western side and steep scarp slopes on its south-eastern 

 side. The latter are in places very bold and precipitous, and show clear-cut 

 sections. Especially is this the case at the Horseshoe Cliff, about a mile 

 below the gorge, where the north-western slope of the downs has been scored 

 by a deep washout, and the strata are clearly exposed for 500 ft. 



In the lower part of the river-gorge there is a most interesting occurrence 

 of the lower members of the Cretaceo-Tertiary series, analogous to Ihat seen 

 in the Waipara and Weka Pass sections. These beds when followed along 

 the strike run in the direction of the greywacke mass of Mount Grey ; and 

 unless they turn round on approaching it, as they do on the north-east slope 

 of the mountain, the junction between the two sets of beds will in all pro- 

 bability be a fault contact. To the west of the gorge, however, the junction 

 between the two sets of beds is a normal unconformity. 



The following is a description of the beds here exposed, the sequence 

 being in ascending order : — 



1. Greyish sands and sandy shales, glauconitic. concretionary in places, 

 and stained with sulphur ; succeeded by light-coloured argil- 

 laceous and slightly glauconitic sands — all striking north-east and 

 dipping south-east at 45°. 



