Sheewsbury. — On the Auckland Volcanoes. 517 



blance is seen, the latter rock showing the same peculiarity in 

 the arrangement of circular augite crystals in the base. The 

 groundmass, indeed, in these two rocks is almost identical. 

 It is very probable, therefore, that the Northcote tuff-craters 

 and Lake Takapuna are closely connected, and drew their 

 supplies of lava from the same point. 



A more interesting peculiarity occurs in a small basalt- 

 dyke which has been exposed in one of the scoria-pits at 

 Mount Eden. This rock shows to the naked eye numerous 

 small ci-ystals which have the shape and general appearance 

 of olivine, but are black. The microscope shov?s that they 

 are in reality olivine crystals almost entirely filled with 

 magnetite grains and dust. This magnetite is partly scattered 

 irregularly through the crystals, partly arranged in parallel 

 rows, and in lines crossing one another at right-angles. It 

 may have been produced in either of two ways. It may be 

 the result of weathering : the first stage of decomposition 

 would then be marked by the production of ferric oxide, which 

 on further weathering would be converted into magnetite. 

 Examples of this are frequently met with in volcanic rocks, 

 the olivine being altered round the margin and along the 

 cracks, and even for some distance on either side of them, into 

 red ferric oxide, which sometimes shows a fibrous arrange- 

 ment similar to that assumed by the magnetite in the present 

 instance. A further change takes place in the centre of the 

 cracks and at places most exposed to decomposing influences, 

 and the red ferric oxide becomes at these places converted 

 into the black oxide or magnetite. A crystal of olivine under 

 these circumstances shows the fresh unaltered mineral in its 

 central parts, changing to red oxide of iron towards the edges 

 and fissures, which in its turn changes to black oxide in the 

 cracks themselves and round the margin of the crystal. It is 

 more probable, however, that the magnetite in this case w^as 

 formed at the time of the cooling and incipient crystallization 

 of the lava, and is not the result of weathering, for the olivine 

 in question does not cont 'n any of the red oxide, but, on the 

 contrary, the crystal subs ^ance between the grains of magne- 

 tite, as well as the spaces clear of them, are fresh and quite 

 unaltered. It seems, therefore, that this magnetite is not a 

 secondary product, but began to crystallize out before the 

 olivine, and was subsequently caught up and included by that 

 mineral in the course of growth of its crystals. This view is 

 strengthened by the fact that, although the octahedra are so 

 imperfectly developed that the magnetite is for the most part 

 in the form of mere irregular grains, yet the lines in which 

 the grains are arranged are in many cases those assumed by 

 skeleton crystals of magnetite, such as are frequently met 

 with in basaltic rocks. 



