Thomson. — Geology of Middle Waipara and Weka Fuss District . 341 



A still more recent revival of erosion is shown by the existence of small 

 channels, a foot or two in breadth and depth, within the rejuvenated por- 

 tions of the small streams draining from the back of the cuestas of the 

 Mount Brown and Mount Donald beds. This revival, in the opinion of 

 Cotton,* may be due to the quicker run-off of storm- water owing to the 

 eating-down of the original plant-covering by stock and rabbits, or to 

 reduction of the plant-covering by slight desiccation of climate. 



Caves and underground courses in the limestones are not a prominent 

 feature of the area. Shelters caused by overhanging bluffs of Weka Pass 

 stone are common, and are also developed in places in the Mount Brown 

 limestones. The back slopes of the Weka Pass stone cuestas are marked 

 by narrow fissures, generally parallel to the strike, which are mostly choked 

 with' clay and debris to a depth of 8-10 ft. They are well displayed on the 

 cuesta west of the Waipara River limestone gorge, and on the adjoining 

 " grey marls " there are three large sink-holes occupied by lagoons. Sink- 

 holes are also present in a similar position in the angle between the Weka 

 Creek and the Weka Pass Stream. A persistent series of caves, said to be 

 of considerable depth and extent, occurs at the foot of the Amuri limestone 

 cuesta near Onepunga, just in front of the outcrop of the Weka Pass stone. 

 The only underground course I have observed is in the Omihi Creek, north- 

 east of Mount Donald, where the stream flows for a short distance under- 

 ground in the Weka Pass stone. Springs and marsh}- areas are well dis- 

 played in many places and at various horizons where loose sands rest on 

 more impervious rocks — e.g., on the hillside below Trounce "s quarry, near 

 Waikai«e, where sands below the Amuri limestone and giauconitic mudstone 

 rest upon the Waipara greensands. 



DETAILED STRATIGRAPHY. 

 PiRIPAUAN. 



The Piripauan rocks of the district include all those Notocene rocks 

 below the Amuri limestone and the marls, mudstones, or sandstones down 

 into which it passes. All the fossils obtained from this group have been 

 determined as Upper Senonian, including the Reptilia of the " saurian 

 t)eds," and the presence of Reptilia in the overlying Waipara greensands 

 justifies their inclusion also in the Piripauan. The rocks are nearly 800 ft. 

 thick in the Middle Waipara district, but they are not moTe than 150 ft. a 

 few miles east of the Weka Pass. The thinning- out is due to overlap, 

 since in the eastern end of the district the sequence commences with the 

 uppermost division, the Waipara greensands. Within the Piripauan no 

 unconformity has been detected by any observer. The beds in most cases 

 pass gradually into one another, and the divisions recognized are only for 

 convenience of description. 



Coal-measures anU Ostrea Beds. 



In the western part of the district a bed formed mainly of black oyster- 

 shells is very persistent, but as coal-seams lie sometimes above and some- 

 times below it it is, conveniently included with the coal-measures. The 

 clearest sections are those of the Doctor's Gorge, Waipara River, and in the 

 various tributaries of Boby's Creek. 



r 



* Personal communication. 



