Shrewsbury. — On the Auckland Volcanoes. 367 



The tuff-cones were evidently formed prior to the scoria-cones, 

 and, as stated by Hoclastetter, appear to have been formed 

 under water, for, where cut across by roads or opened up by 

 gravel-pits, their materials are distinctly stratified. A very 

 good instance of this bedded structure can be seen at Lake 

 Takapuua, where a scoria-pit in the northern wall of the 

 crater shows a thick bed of black scoria, succeeded by a bed 

 of sand and scorias mixed. This sandy bed is stratified and 

 banded, and has every appearance of having been laid down 

 in water. When the volcanic action commenced, therefore, 

 the isthmus, or parts of it, must have been under the sea. We 

 will return later on to a discussion of the condition of the 

 land at the time of the first eruptions. 



What is the reason of the essential difference between the 

 tuff-cones and the scoria-cones? Why is it that the former 

 are so much wider and flatter, and contain so much more non- 

 volcanic material, than the latter? The reason, I think, is 

 twofold. In the first place, the earliest eruptions, by which 

 the tuff-cones were produced, would naturally be more violent 

 than those succeeding — clearing away obstructions, rending 

 and reducing the superincumbent rocks to fragments, and 

 opening up a way by which the subsequent eruption of vol- 

 canic material would be comparatively easy. Such violent 

 paroxysmal outbursts, as stated by Judd, produce fiat cones, 

 of low elevation, with wide craters, such as these tuff-craters ; 

 while the steeper and smaller scoria-cones are the result of 

 more moderate but long-continued volcanic action. In the 

 second place — and in this probably lies the chief reason of the 

 difference — the presence of water in abundance would largely 

 increase the violence and suddenness of the first eruptions, for 

 we know that the chief factor in volcanic explosions is steam, 

 and we can readily imagine that the sea-water would not only 

 enter the fissures formed at the commencement of volcanic 

 action, but would also percolate through and fill the pores of 

 the sandstones, &c., of the Waitemata beds. The conversion 

 of this water into steam, when the tension became so great as 

 to overcome the weight of the overlying rocks and water, 

 would not only add to the sudden force and intensity of the 

 eruption, but would also cause the comminution of the sand- 

 stones, and thus account for the large amount of non-volcanic 

 material in these earliest-formed craters. 



Some of the tuff-craters are situated in close proximity to 

 the sea-shore, and in such cases we generally find that the side 

 of the crater nearest the sea has given way, and the crater has 

 become filled with mud and sediment carried in by the sea. 

 This is seen at the Orakei and Panmure basins, which at low 

 water are mere mud-flats and mangrove swamps, connected 

 with the sea outside by narrow channels cut through the mud 



