Speight. — Structural and Glacial Feature* of Hurunui Valley. 95 



Basement Rocks of the Area. 



The characteristic basement rock of the region is a greywacke such as 

 is typically developed in the mountains of Canterbury farther south. This 

 is usually of the hard grey facies, but slaty greywackes also freely occur, 

 which break down under the weathering agencies into clay and form a 

 covering on the mountain-slopes. On these forest once became thoroughly 

 established, but it has been largely destroyed in the higher parts of the 

 riv«r-valley by the grass fires of settlers. The greywackes have a general 

 north-easterly strike, with local variations. Beds of dark-red slaty shale 

 also occur, as well as occasional outcrops of volcanic rock. Basalts and 

 andesites occur near Lake Sumner on the Crawford Range, and there is 

 in the Canterbury Museum a specimen from the same area marked " eurite " 

 by Hutton. Basalt pebbles occur in the Seaward River, a tributary coming 

 in from the south about three miles below the junction of the two branches, 

 and similar rocks occur in position between it and the Waitohi River. 



The most interesting occurrence is near the Mandamus. About a mile 

 above its junction with the main stream a massive intrusion of augite 

 syenite occurs in the greywacke. This has a general north-easterly trend, 

 and it appears to have the character of a sill, being approximately parallel 

 to the dip and strike. Its thickness is more than 200 ft., and it extends 

 over a mile in length. Associated with it are trachyte dykes, and flows of 

 augite andesite occur in close proximity. Hutton was of the opinion that 

 the syenite represented the core of a volcano of which the andesite was the 

 effusive representative. But angular fragments of the syenite are found in 

 the andesite, in some places in considerable quantity, so that the intrusion 

 of the syenite was evidently anterior to the andesite. We have therefore, 

 in order of time, (1) greywacke, (2) syenite, and (3) andesites. In this 

 district, too, there are basic volcanic tuffs having in their higher levels a 

 calcareous tufa facies passing into a true limestone; but the volcanic beds 

 are much better developed to the north-east, in the Pahau and Culverden 

 districts, where there are interstratifications of volcanic material between 

 beds of limestone. The occurrence at the Mandamus points to several 

 periods of vulcanicity, the channel opened by the syenite affording a passage 

 for later magmas. 



Younger Rocks. 



Volcanic rocks have exerted little effect on the area covered by the river- 

 basin, in which greywacke is now by far the most dominant member ; but 

 at one time the lower parts of the valley were covered with a veneer of 

 Tertiary sediments, remnants of which are still to be found. These later 

 beds have all a north-easterly strike, so that they cut across the river at a 

 average angle of 45°, and at present they occupy separate compartments 

 of the valley, cut off from adjacent ones by ridges of greywacke. These 

 isolated areas are as follows : (1) In the Dove River, a tributary of the 

 Mandamus coming in on the east ; (2) in the Hurunui- Waiau basin ; 

 (3) in the Waikari-Kaiwara basin, or rather trench ; (4) in the Greta- 

 Cheviot basin. The special features of these may be taken in turn. 



(1.) The Dove River Area. 



The Dove River basin is important, not from its size, but because it 

 gives an indication of the origin of the landscape features of a considerable 

 area of hill country forming a kind of platform or terrace at the base of the 



