Uttley. — Volcanic Rocks of Oainaru. 



115 



H„0 



0-79 



0-52 



The specific gravity of the tachylytic selvage was found to be 2-74, 

 while that of the interior of the pillow was higher, 2-83. This is in accord- 

 ance with Hutton's results for the similar rock at the lighthouse, his figures 

 being 2-72 and 2-80 respectively (1887, p. 416). 



The pillow-lavas at Awamoa Creek (near Deborah) and at the lighthouse 

 near the breakwater show much the same character. In the field there is 

 noticeable on each pillow a distinct black resinous-looking selvage about 

 1 in. in thickness, while the central portion has all the characteristics of 

 a basalt. 



A number of sections have been made from these rocks, and as far as 

 examined they present several features of some interest : it is possible to 

 trace the gradual changes from a basaltic glass with a few phenocrysts, 

 undoubtedly of intratelluric origin, to a holocrystalline dolerite, in which 

 the phenocrysts display a similar attitude towards a completely crystalline 

 ground-mass. 



The extreme edge of the tachylyte selvage consists of light brownish- 

 yellow glass in which small porphyritic labradorite crystals and larger crystals 

 of olivine occur as phenocrysts. Skeleton crystals of a basic feldspar also 

 occur, and minute granules of magnetite are scattered sparingly through 

 the glass, which is irregularly fissured. The olivine is almost invariably 

 corroded by the ground-mass, and some of the crystals are penetrated deeply 

 by the glassy base, this being particularly noticeable in the case of the larger 

 ones. Flow-structure is indicated by the parallel alignment of the feld- 

 spars. In a section cut farther from the edge the feldspars are bordered 

 by a dark-brown fringe, giving a shadowy extinction between crossed nicols. 

 Under a high power this resolves itself into minute spicules arranged radially 

 around the feldspars. Where the growth has occurred around a minute feld- 

 spar there is an approach to a spherulitic arrangement, and an indistinct 

 black cross is seen when the nicols are crossed. It is noticeable that the 

 glass is not bleached on the periphery of this aggregation of crystallites, 

 as is frequently the case in spherulitic tachylytes. Near the centre of the 

 pillow the glass has completely disappeared, and the ground-mass consists 

 of this fibrous brown material shot through with innumerable skeleton 

 feldspars in all stages of growth, the whole enclosing the same minerals 

 that were developed porphyritically in the tachylitic variety of the rock. 

 Still nearer the centre of the spheroid the ground-mass becomes lighter 

 in colour, and rods' of magnetite are plainly distinguishable in a faintly 



