Gilbert. — Geology of Waikato Heads District. HI 



5. Volcanic ash, breccia, and basaltic lava. The marginal portions of the 



plug contain included fragments of the calcareous beds through which 

 it was extruded. The volcanic-ash bed contains large fragments of 

 the underlying calcareous beds, and varies from a coarse ash in the 

 north to a breccia or agglomerate as it approaches the neck of the 

 volcano. These beds are distinctly unconformable to the underlying 

 fossiliferous sands. 



6. Stream-bedded sands, 30 ft. thick, follow ; they include a band of lignite 



8 in. thick. In proximity to agglomerate which is above the vol- 

 canic lava-plug they appear locally to overlie beds of included tuff of 

 which the upper limit is a sharply marked erosion-plane coincident 

 approximately with the upper level of the yellow fossiliferous sands. 



7. Brown sands (30 ft.)' follow, whose lower layers are horizontally bedded, 



whilst higher up they are composed of peculiar lenses encrusted by 

 limomtized ironsand. The thin encrusting layers show an inter- 

 lacing tendency typical of wind-blown sands where the winds change 

 direction frequently, and so form confused series of ripple-marks. 

 It is not easy to explain why the encrusting layers alone should 

 become limonitized, leaving the sand between loose and unaltered. 



8. Pumice-bed (10 ft. to 20 ft.). This is a white, light, slightly plastic clay 



band, its very thinly bedded nature indicating deposition in the fairlv 

 still water of a swamp or lake. It is undoubtedly a fine pumice, 

 enclosing large fragments of the same material. Non-pumiceous silts 

 of irregular thickness replace the pumice to the south, above the 

 volcanic conglomerate which covers the remnant of the lava-plug. 



9. 200 ft. of brown, oxidized, wind-blown sands rich in limonite concretions. 



The Kawa Pumice-hed in Relation to the Waikato River. 



The occurrence of this bed of pumice-sand, containing coarser fragments 

 of pumice, 180 ft. above high water, so far south of the mouth of the Waikato 

 River makes one hesitate to ascribe its origin to transport by that river 

 of material from the pumice plateau through which it flows for so much of 

 its upper and middle course. No other origin, however, readily suggests 

 itself, whilst this theory has several facts to support it : — 



(1.) There is no other visible source whence the material may be derived. 



(2.) The characteristic deposits made by the Waikato in the Bay of 

 Plenty district and in the Hauraki Plains are largely rhyolitic pumice-silt 

 which resembles the Kawa deposit. 



(3.) Not only has the coastal area risen, but the whole country to the 

 east and south-east as far as the middle Waikato basin, including the 

 southern portion of the Hauraki Plains, across which the river flows in a 

 north-westerly direction, has also been elevated with reference to sea-level 

 since the course of the Waikato River was diverted from its old channel 

 leading through the Hinuwera Valley to the Hauraki Gulf. There has thus 

 been regional uplift. At the point below Maungatautari Gorge where the 

 river enters the middle Waikato basin the surface of the plain is 300 ft. 

 above sea-level. According to Henderson (1918, p. 60) this plain was formed 

 by loose pumice of fluviatile origin whilst the land was depressed. About 

 this time also the river changed its course from the Hinuwera Valley to the 

 north-west across its own alluvial plain. (See Henderson 1918, pp. 112-15; 

 and Cussen, 1889, p. 409, and 1894, pp. 401-10). The pumice of the Kawa 

 beds must have been brought down at that time and deposited in a depression 

 forming a swamp on the borders of a large estuary or low-lying coastal land 

 such as then existed. When elevation ensued the tendency would be for 

 the river to deepen its bed, and this has been done across the middle Waikato 

 basin, the deepening here corresponding approximately to the uplift of the 



