Shrewsbuey. — On the Auckland Volcanoes. 371 



are of frequent occurrence. Inclusions of olivine (nodules) do 

 not occur in the scoria-cones, but inclusions of clay, sand- 

 stone, &c., and of silica are not infrequently met with. The 

 former are fragments of the Waitemata beds which have been 

 caught up by the lava in its expulsion as scoriae, and altered 

 by the heat to which they have been subjected. This altera- 

 tion has been carried to very varying degrees : in some cases 

 the fragments have been greatly hardened, have assumed 

 various shades of grey, red, and black, and have acquired a 

 porcelain-like appearance, whence they are known as porcel- 

 lanites ; in others very little change has been produced, and 

 the inclusion consists of soft sandstone, just like the unaltered 

 rock of the Waitemata beds. They are generally somewhat 

 rounded, and the exterior is either partly or wholly covered 

 with adhering scoria, sometimes in thick lumps, sometimes as 

 a thin semi-vitreous coating which can usually be readily 

 cracked off. As a rule the lava has not penetrated to any 

 extent ; some of the porcellanites, however, show minute veins 

 of penetrating lava. A microscope section which I have pre- 

 pared from the outer part of such a porcellanite shows in an 

 interesting manner the gradation from ordinary basalt on the 

 exterior to an almost perfect glass at the furthest limit of the 

 vein. Occasionally these porcellanites display a very interest- 

 ing columnar or prismatic structure, due to contraction on 

 cooling. Since they are more or less spherical in form, and 

 contraction-jointing took place at right-angles to the surface of 

 cooling, the columns or prisms radiate from the centre. They 

 can be more or less readily separated from one another, giving 

 rise to a number of curved and tapering fragments. 



The inclusions of silica are smaller and less common than 

 those of the preceding class. They are fragments, generally 

 somewhat rounded, but sometimes angular, of nearly pure 

 quartz, in the form of a crystalline-granular aggregate, 

 crumbling almost with a touch. Except when stained with 

 ferric oxide, they are colourless or white. They generally, like 

 the porcellanites, show a loosely-adhering coating of basalt ; 

 many also contain small beads and threads of glassy basalt, 

 which has penetrated them from the lava as it surrounded them. 

 It is difficult to account for the origin of these quartz-inclusions. 

 It is improbable that they are derived from the slates and 

 greywackes which underlie the Waitemata beds, for these do 

 not, where exposed near Auckland, contain free quartz, except 

 in minute quantities ; and, moreover, no fragments of the 

 slates themselves are found in conjunction with the quartz, 

 as we should certainly expect to find if this was the origin of 

 these fragments. The Waitemata beds themselves contain no 

 free quartz or quartz-veins. Whence, then, is this quartz 

 derived ? To this question I am unable to give any satisfac- 



