Gilbert. — Geology of Waihato Heads District. 101 



A Suggestion of Origin of the Sand-dunes and of the Lignite-beds. 



North of Manukau Harbour is a strongly resistant coast-line which has 

 retrogressed considerably owing to wave-attack. Similarly, south of the 

 Waikato River all evidence points to considerable sea-cliff recession. The 

 writer's belief touches entirely new ground, and it is this : that regularity 

 of coastal outline was reached between Waikato Head and Manukau North 

 Head hy spit or perhaps barrier-beach formation in the not distant past, 

 when the relative level of sea and land approximated the present, and 

 that a great estuary of the Waikato River was formed behind this barrier, 

 in which pumice-silts were deposited and bands of lignite formed. This 

 beach supplied the material raised by wind into lofty sandhills, which 

 have been cut into by the waves as the shore-line advanced towards 

 maturity. 



Mr. J. A. Bartrum, in mentioning to me that this accorded with his own 

 view, called my attention to a fact which I have since been able to confirm — 

 namely, that there are similar sand-ranges west of Helensville, going north 

 along the western margin of the Kaipara Harbour. He further pointed out 

 that in that district there is every evidence of former uplift in elevated 

 erosion-plains. 



This theory of the origin of the sand-ranges with an extensive estuary 

 of the Waikato River behind them readily accounts for the origin of the 

 pumiceous silts and the lignite bands west of the ranges, for it is believed 

 that the silts were formed in the estuary by the deposition of fine material, 

 largely pumiceous, brought down by the Waikato River from the great 

 pumice lands of the middle of the Island. There are pumice-silts at Mangere, 

 opposite Onehunga, at Otahuhu, and near Drury and Papakura. They 

 are thus very widespread round the shores of Manukau Harbour, the waters 

 of which cannot have supplied the material. The Waikato River, then, 

 appears to be the only source of origin that can satisfactorily account for 

 these silts. Well-borings in various places around Waiuku and the Aka- 

 aka Swamp support the view that an extensive estuary existed. The 

 lignites at and above high-water mark along the Waiuku Creek, and exposed 

 in railway-cuttings between Otahuhu and Papakura, would be formed in 

 this estuary by the accumulation of vegetable material in the swamps. 



The two lignite bands at the foot of the sand-dunes on the coast contain 

 fragments of wood, undecomposed or slightly carbonized, and, amongst 

 other vegetable remains, the abundant long leaves of the raupo (Typha 

 angustifolia) . They were probably formed in shallow lakes or lagoons 

 occurring in the hollows of the sandhills in their early stages, just as the 

 remains of similar vegetation are accumulating at the present time around 

 the swampy raupo-covered margins of Lake Pokorua, north-west from 

 Waipipi, and other lakes even in the shifting sand-dunes near the Waikato 

 River. It is possible, however, that they had an origin similar to that of 

 the Waiuku bands — that is, in the swamps marginal to the early Waikato 

 estuary, which have since been covered up by the inland advance of the 

 dune-belt, and then re-exposed by sea-cliff recession in conformity with the 

 general retrogression of the coast both north and south of this area. 



Sub-recent Oscillations of Level : Origin of Manukau Harbour. 



The bands of lignite exposed at frequent intervals along the banks of 

 Waiuku Creek are either at or slightly above high-water mark, and are 

 covered to a depth of from 5 ft. to 20 ft. by silts. They thus furnish 

 evidence of sub-recent minor oscillations of the district. In the arm of 



