366 Transactions. — Geology. 



Art. XXXI. — The Auckland Volcanoes. 



By Hugh Sheewsbury, M.A. 



[Read before the Auckland Ins/itute, 2nd November, 1891.] 



Plate XXXV. 



The isthmus which separates the Waitemata and Manukau 

 Harbours, and upon which stands the City of Auckland, has 

 an average breadth of six miles ; but at its narrowest part, 

 between the eastern shore of the Manukau and the Tamaki 

 River, it is not more than a mile and a half in width ; and, 

 again, between the Whau River and the Manukau its breadth 

 is only about two miles. Small as this tract of land is, how- 

 ever, it is thickly studded with extinct volcanoes, there being 

 no less than sixty-three separate jjoints of eruption within a 

 radius of ten miles, in many places so close together as to 

 merge into one another. The greater number of these volcanic 

 cones are in a very perfect state of preservation. It is true 

 that many of them have been deeply terraced by the Maoris 

 for purposes of fortification ; many also have been cut into 

 to obtain supplies of road-metal ; but from weathering and 

 denudation these hills have suffered little, and are remarkably 

 well preserved. They present the form of cones of low 

 altitude, Rangitoto, the highest of them, being only about 

 920ft. in height, and the slope of their sides being about 30° 

 or 40°. The majority of them are dome- or mound-shaped 

 rather than conical. Classifying them according to their 

 mode of formation, we may divide them into three classes : 

 (1) tuff cones and craters, (2) scoria cones and craters, 

 (3) lava cones. 



1. Ttiff Cones and Craters. — Instances of these are Lake 

 Takapuna and the Orakei and Panmure basins. They are 

 readily distinguished from the scoria,-cones by their shape, 

 being wider and flatter, with much larger craters. The 

 material composing them is not, as in the case of the scoria- 

 cones, entirely scoria and lava, but consists of a mixture of 

 sand and grit of non-volcanic origin, derived from the Waite- 

 mata beds, with volcanic blocks, scoritE, lapilli, and ash, in 

 some cases the former, in others the latter, class of material 

 predominating. Some of the tuff-craters have been partly 

 filled up by scoria-cones thrown up by subsequent eruptions — 

 for example. Mount Welhngton and the North Head — and it 

 is probable that many of the scoria-cones stand upon older 

 tuff-craters, which, however, are hidden from view by the great 

 quantities of lava and scoria ejected by the later eruption. 



