368 Transactions. — Geology. 



by the ebb and flow of the tide. There are two of these 

 crater-basins near Northcote, which are remarkable from the 

 fact that they contain, in the material of their walls, certain 

 olivine nodules which are extremely interesting as affording a 

 parallel to the nodules of that mineral found in some Tertiary 

 basalts of Europe. They are small — the largest I have found 

 measure only about l|-in. by lin. They fracture with an un- 

 even, crystalline-granular surface, and are sometimes so loosely 

 coherent as to be easily crumbled down to a coarse powder ; 

 most, however, are harder and more compact, requiring a pair 

 of pincers or a hammer and chisel to break them. The olivine 

 is of a pale yellowish-green colour, and remarkably fresh. In 

 some of the nodules a few of the grains are coloured a dark- 

 grey, almost black, by vast numbers of grains and fine thread- 

 like or needle-shaped inclusions of magnetite and dark- 

 coloured glass. Most of these inclusions are so minute as to 

 appear as short, fine, hair-like bodies when examined with a 

 magnifying-power of 300 diameters, and are arranged in 

 parallel lines or rows closely crowded together. More in- 

 teresting, however, is the appearance of fluid inclusions in this 

 olivine. Their presence points to the deep-seated origin of 

 the nodules, and so exactly illustrates the words of Teall in 

 his "British Petrography" that these w^ords may be quoted 

 here : "If we consider the distribution of fluid inclusions in 

 the different classes of rocks, we are struck by the fact that 

 they are especially characteristic of the plutonic rocks, such 

 as gabbro, diorite, and granite, and the crystalline schists. 

 They are rare or absent in rocks of the volcanic group. . . 

 We do occasionally find glass and stone inclusions in the 

 minerals of certain granites, and fluid inclusions in those of 

 volcanic rocks, as, for instance, in the olivine and leucite of 

 certain lava-streams; but it nmst be remembered that in these 

 exceptional cases the minerals in question have probably been 

 developed before the actual eruption of the lava." This is just 

 what appears to have occurred here. The olivine nodules 

 must have crystallized out or segregated from the magma 

 some time before it was erupted — while it was still deej) below 

 the surface — and were afterwards ejected along with the 

 scoria and ash formed by the comminution of the surrounding 

 magma. That they are segregations from the basalt itself, and 

 not inclusions of foreign matter, is, I think, almost certain ; 

 they do not contain penetrating veins of basalt, nor possess a 

 thin easily-removed coating or shell of basalt, as exhibited by 

 the foreign inclusions occurring in the scoria which will be 

 presently described. Moreover, the olivine is exactly similar 

 to that occurring so abundantly in smaller crystals in all the 

 Auckland basalts. Their origin, therefore, is to be explained 

 by the " segregation hypothesis "put forward by Eoth, Eoseu- 



