378 Transactions. — Geology. 



These, hov/ever, are mere local variations in form or struc- 

 ture. The constituents which build up the lava, from 

 whatever point erupted, are invariably the same — felspar, 

 pyroxene, olivine, and magnetite ; and, moreover, it appears 

 that the felspar may in every case be referred to the labra- 

 dorite-anorthite group, while the pyroxene probably occurs 

 throughout in the form of augite only. No other mineral 

 enters into the composition of these basalts at any point : on 

 the contrary, the accessory ingredients found in some basalts 

 are not met with in this rock. The internal structure of the 

 lava is similar at all points at which it has cooled and con- 

 solidated under similar conditions, and what difterences there 

 are lie not in the constituents themselves, but in their form 

 and mode of development. Moreover, with regard to such 

 differences, we have seen that no variety characterizes the 

 lava from any focus of eruption, but that the differences in 

 texture, relative arrangement, and proportions of the com- 

 ponent crystals, are observable in different parts of the lava 

 from one and the same volcano, and pass by insensible grada- 

 tions into one another. We have seen also that the basalt 

 which occurs in the form of scoriae and fragmentary material 

 is the same rock as that which flowed as streams of lava ; 

 in short, that in composition, colour, and general outward 

 appearance, as well as in microscopic structure, the materials 

 ejected from all the points of eruption so closely resemble 

 one another that the lava in all cases may be considered 

 identical in origin. We have seen that the lava is basic — so 

 much so, in some cases, that the olivine has separated out, or 

 crystallized, to an unusual extent before the actual eruption of 

 the lava, and in at least one instance was ejected in the form 

 of nodules. The conclusions to be drawn from these facts 

 are that the eruptions are not distinct, but closely connected 

 with one another, and belong to one period — that, in fact, they 

 originated from a common point or focus of activity ; and, 

 further, that this focus was not situated near the surface, but 

 at some depth — possibly beneath the oldest rocks in the neigh- 

 bourhood. All the volcanoes were probably connected with 

 one and the same reservoir of molten rock, which, instead of 

 escaping at a single point with one great explosion, was ex- 

 pelled through a great number of minor vents. 



x\ll the points of eruption may be included in an elliptical 

 area, the long axis of which has a general north-east and 

 south-west direction. Moreover, a line joining the principal 

 volcanoes — Rangitoto, Mount Eden, One-tree Hill, Mount 

 Elliot (Mangare), and the three volcanoes situated in a line 

 with one another, and known as Moerangi and Maungatake- 

 take — also trends to the north-east. This line, if produced in 

 a north-easterly direction, joins the " line of volcanic activity" 



